Grey flatfish crossing river (4)
I believe the answer is:
drab
'grey' is the definition.
(thesaurus)
'flatfish crossing river' is the wordplay.
'flatfish' becomes 'dab' (I've seen this before).
'crossing' is an insertion indicator.
'river' becomes 'r'.
'dab' going around 'r' is 'DRAB'.
(Other definitions for drab that I've seen before include "Cheerless, shabby" , "Colourless kind of bard" , "Dull, uninteresting" , "woman with slatternly habits" , "Shabby; a slattern (arch.)" .)