Bond formed by boy in group (7)
I believe the answer is:
sealant
'bond' is the definition.
(I've seen this before)
'boy in group' is the wordplay.
'boy' becomes 'alan' (I've seen this before).
'in' indicates putting letters inside.
'group' becomes 'set' (both can mean a collection).
'alan' placed within 'set' is 'SEALANT'.
'formed by' is the link.
(Other definitions for sealant that I've seen before include "Substance filling gap to prevent leakage" , "Material to prevent leakage" , "Mastic maybe" , "Impervious gap filler" , "Material for making airtight or watertight" .)