Old boy with fly catches first of common carp (6)
I believe the answer is:
object
'carp' is the definition.
(to carp is to complain or object to something)
'old boy with fly catches first of common' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'with' means one lot of letters go next to another.
'fly' becomes 'jet' (jetting is a kind of flying).
'catches' is an insertion indicator.
'first of common' becomes 'c' (jst letter of 'common').
'jet' placed around 'c' is 'ject'.
'ob'+'ject'='OBJECT'
(Other definitions for object that I've seen before include "Protest; purpose; thing" , "Something that can be seen and touched" , "Idea" , "part of sentence" , "Aim or purpose" .)