Keen to help a number inside (4)
I believe the answer is:
acid
'keen' is the definition.
(I've seen this before)
'help a number inside' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'a number' becomes 'c' (C is an example**).
'inside' means one lot of letters goes inside another.
'aid' enclosing 'c' is 'ACID'.
'to' is the link.
(Other definitions for acid that I've seen before include "scathing" , "So-called drug" , "Sour tasting" , "Tart - keen - sort of house party" , "Drug LSD (colloq.)" .)