Humble when back in Cambridge collecting degree (5)
I believe the answer is:
abase
'humble' is the definition.
(synonyms)
'when back in cambridge collecting degree' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'back in' indicates one should take the final letters (the 'back' of the word).
'collecting' is an insertion indicator.
'degree' becomes 'BA' (Bachelor of Arts).
The last letter of 'cambridge' is 'e'.
'as'+'e'='ase'
'ase' enclosing 'ba' is 'ABASE'.
(Other definitions for abase that I've seen before include "Lower" , "Humiliate" , "Belittle, degrade" , "Cast down" , "what 11 [REPRIMANDS] are meant to do" .)