Popular source of customary string for bow (7)
I believe the answer is:
incline
'for bow' is the definition.
(I've seen this before)
'popular source of customary string' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'source of' suggests taking the first letters (the start - or source - of the word).
'string' becomes 'line' (line is a kind of string).
The initial letter of 'customary' is 'c'.
'in'+'c'+'line'='INCLINE'
(Other definitions for incline that I've seen before include "lean forward" , "Lean, slope" , "Turn" , "Tend - slope" , "Bend forwards" .)