Plenty of cotton socks (4)
I believe the answer is:
tons
'plenty' is the definition.
(I've seen this in another clue)
'of cotton socks' is the wordplay.
'of' indicates a hidden word.
'TONS' is hidden in the letters of 'cotton socks'.
(Other definitions for tons that I've seen before include "Each is equivalent to 2240 pounds" , "A lot" , "Loads (sl.)" , "Lots" .)