Previously seen in convention centre (4)
I believe the answer is:
once
'previously' is the definition.
(I've seen this before)
'in convention centre' is the wordplay.
'in' says the answer is hidden in the clue.
'ONCE' can be found hidden inside 'convention centre'.
'seen' acts as a link.
(Other definitions for once that I've seen before include "In former times" , "Cone (anag.)" , "One-time - one time" , "Some time back" , "Long ago" .)