Dull river crossed by fish (4)
I believe the answer is:
drab
'dull' is the definition.
(similar in meaning)
'river crossed by fish' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'crossed by' is an insertion indicator (inserted letters are crossed or covered by others).
'fish' becomes 'dab' (dab is a type of fish).
'r' put inside 'dab' is 'DRAB'.
(Other definitions for drab that I've seen before include "Dull light brown colour" , "Lacking brightness or colour" , "Dismal < slattern" , "Dull and monotonous" , "Grey or light brown cloth" .)