Complain as old boy caught in stream (6)
I believe the answer is:
object
'complain' is the definition.
(to object to something)
'old boy caught in stream' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'caught' becomes 'c' (cricket abbreviation).
'in' indicates putting letters inside.
'stream' becomes 'jet'.
'c' inserted inside 'jet' is 'ject'.
'ob'+'ject'='OBJECT'
'as' acts as a link.
(Other definitions for object that I've seen before include "oppose" , "deplorable spectacle" , "Protest; purpose; thing" , "Thing or target" , "Voice disapproval; goal" .)