Crew after fish — is that plain? (9)
I believe the answer is:
pikestaff
'that plain?' is the definition.
(I've seen this before)
'crew after fish' is the wordplay.
'crew' becomes 'staff' (I've seen this before).
'after' says to put letters next to each other.
'fish' becomes 'pike' (pike is a kind of fish).
'staff' put after 'pike' is 'PIKESTAFF'.
'is' acts as a link.
(Other definitions for pikestaff that I've seen before include "Mediaeval lance" , "As plain as this is obvious" , "Shaft, proverbially plain" , "wooden rod?" , "Proverbially plain handle of old weapon" .)