Ingredient of fine Guinness, perhaps (6)
I believe the answer is:
factor
'ingredient' is the definition.
(I know that ingredient can be written as factor)
'fine guinness perhaps' is the wordplay.
'fine' becomes 'f' (abbreviation).
'guinness perhaps' becomes 'actor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'f'+'actor'='FACTOR'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for factor that I've seen before include "Seven is one for thirty five" , "for a consideration?" , "Circumstance that influences result" , "Of 15, 3 or 5" , "one to be taken into account" .)