Hence it's in the pergola (4)
I believe the answer is:
ergo
'hence' is the definition.
(I've seen this in another clue)
'in the pergola' is the wordplay.
'in' says the answer is hidden in the clue.
'ERGO' is hidden within 'pergola'.
'it's' acts as a link.
(Other definitions for ergo that I've seen before include "Consequently" , "Therefore - what a goer" , "It follows" , "for this reason" , "Thus" .)