Sugar, maybe, fed to group -- that fills a gap (7)
I believe the answer is:
sealant
'that fills a gap' is the definition.
(substance for sealing gaps)
'sugar maybe fed to group' is the wordplay.
'sugar maybe' becomes 'alan' (I can't explain this - if you can you should believe this answer much more).
'fed to' means one lot of letters goes inside another.
'group' becomes 'set' (both can mean a collection).
'alan' inserted into 'set' is 'SEALANT'.
Can you help me to learn more?
(Other definitions for sealant that I've seen before include "Material to prevent leakage" , "Material stopping leaks" , "Impervious gap filler" , "Filler used to close a gap" , "closer?" .)