Firm, small lettuce (3)
I believe the answer is:
cos
'lettuce' is the definition.
(cos lettuce is a kind of lettuce)
'firm small' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'small' becomes 's' (abbreviation - e.g. clothes size).
'co'+'s'='COS'
(Other definitions for cos that I've seen before include "Greek island and type of lettuce" , "Salad ingredient" , "vegetable?" , "Greek island (alternative spelling): Trig abbrev!" , "often associated with sin" .)