Songbird in roof — snug in the middle on top (8)
I believe the answer is:
nuthatch
'songbird' is the definition.
(I've seen this before)
'roof snug in the middle on top' is the wordplay.
'roof' becomes 'thatch' (thatch is a kind of roof).
'in the middle' means to look at the middle letters.
'on top' says to put letters next to each other (down clues only).
The middle letters of 'snug' are 'nu'.
'thatch' put after 'nu' is 'NUTHATCH'.
'in' acts as a link.
(Other definitions for nuthatch that I've seen before include "Tree-climbing bird" , "Singer" , "Tree-climbing songbird" , "Songbird feeding on insects and kernels" , "Bird - hunt chat (anag)" .)