Keen to help a number trapped inside (4)
I believe the answer is:
avid
'keen' is the definition.
(thesaurus)
'help a number trapped inside' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'a number' becomes 'v' (V is an example**).
'trapped inside' indicates putting letters inside.
'aid' placed around 'v' is 'AVID'.
'to' acts as a link.
(Other definitions for avid that I've seen before include "Greatly desirous" , "Fanatical" , "Keen or greedy" , "Greedy like diva" , "Burning" .)