Fish eater, the female consuming a little roach! (5)
I believe the answer is:
heron
'fish eater' is the definition.
'the female consuming a little roach' is the wordplay.
'the female' becomes 'hen' (hen is a kind of female).
'consuming' is an insertion indicator.
'a little roach' becomes 'ro' (I can't justify this - if you can you should believe this answer much more).
'hen' enclosing 'ro' is 'HERON'.
Can you help me to learn more?
(Other definitions for heron that I've seen before include "Graceful grey bird with long neck" , "Large British wading bird" , "Rhone (anag) - bird" , "Big bird" , "One bird" .)