A bird for the architect (4)
I believe the answer is:
This clue is a double definition.
'a bird' is the first definition.
(wren is a kind of bird)
'the architect' is the second definition.
(Sir Christopher Wren)
'for' acts as a link.
(Other definitions for wren that I've seen before include "seafarer" , "Small brown bird 'hunted' on St Stephen's Day" , "Very small brown songbird (4)" , "Jenny?" , "English church architect, d. 1723" .)