Keen to help a number trapped inside (4)
I believe the answer is:
avid
'keen' is the definition.
(both can mean enthusiastic)
'help a number trapped inside' is the wordplay.
'help' becomes 'aid' (synonyms).
'a number' becomes 'v' (V is an example).
'trapped inside' is an insertion indicator.
'aid' placed around 'v' is 'AVID'.
'to' is the link.
(Other definitions for avid that I've seen before include "Greatly desirous" , "Fanatical" , "Keen or greedy" , "Greedy like diva" , "Burning" .)