Here is the relevant entry from the MED. It’s not clear who is glossing it, whether that’s in the manuscript, and if so when the MS was glossed, or whether it’s an addition by a modern editor:
c1225(?c1200) SWard (Bod 34) 12/105: Þe laðe helle wurmes, tadden & froggen..freoteð ham ut te ehnen..ant snikeð [Tit: sniken] in & ut neddren & eauroskes.
Now we only need to start using clumb again! (OE - past clamb/clumbon, ptp geclumben)
The past participle of the OE climban is not geclumben; Old English is not like modern German in this respect. The ge- prefix is either part of the verb in all its principal parts or it is absent in all. It’s really just a superfluous add-on to some verbs that has no purpose that anyone has been able to figure out.
Campbell’s Old English Grammar, sec. 741, classifies climban as a Class III strong verb, which would give its forms as:
climban, *clamb/clomb, *clumbon, clumben
Regarding the vowel change in the preterit, the <a> form of this class is typically found in early Northumbrian texts, although the <o> form is more common. West Saxon and Kentish usage varies, but the <a> tends to be more common.
The DOE says there are three instances of the verb climban in the OE Corpus, but it only includes two citations and my search of the OE Corpus only turns up two. These are:
þa sona on morgen comen ealle þa utlaga mid fela scipe [...] geodon into þe mynstre, clumben upp to þe halge rode
(The sun in the morning comes like an outlaw with much dignity [...] went into the monastery, climbed up to the holy cross)
hit ðurh hrof wædeð, bryceð and bærneð boldgetimbru, seomað steap and geap, stigeð on lenge, clymmeð on gecyndo
(it [fire] moves through the roof, breaks and burns the timbers, stands steep and vaulted, stands in length, climbs in its nature)
The clymmeð spelling is either dialectal variation or scribal idiosyncrasy.