Pipe outside grand garden building? (8)
I believe the answer is:
hothouse
'garden building?' is the definition.
(I've seen this before)
'pipe outside grand' is the wordplay.
'pipe' becomes 'hose' (I've seen this before).
'outside' means one lot of letters goes inside another.
'grand' becomes 'thou' (synonyms).
'hose' placed around 'thou' is 'HOTHOUSE'.
(Other definitions for hothouse that I've seen before include "Plant-forcing building" , "part of the nursery" , "pressurised environment" , "Environment promoting rapid growth" , "Heated building for plants" .)