Some popular gel, yellow mostly (7)
I believe the answer is:
largely
'mostly' is the definition.
(I know that mostly can be written as largely)
'some popular gel yellow' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'LARGELY' is hidden within 'popular gel yellow'.
(Other definitions for largely that I've seen before include "Generally" , "In the main" , "as a rule" , "Allergy (anag.)" , "To a great extent" .)