Make room at the top by trimming trellis (5)
I believe the answer is:
attic
'make room' is the definition.
'attic' can be an answer for 'room' (I've seen this before). I'm unsure of the 'make' bit.
'at the top by trimming trellis' is the wordplay.
'the top by trimming trellis' becomes 'tic' (I can't explain this - if you can you should give a lot more credence to this answer).
'at'+'tic'='ATTIC'
(Other definitions for attic that I've seen before include "Top chamber" , "Lofty room" , "cockloft" , "Athenian - garret" , "Top storey" .)