Period of time presented in gallery earlier (4)
I believe the answer is:
year
'period of time' is the definition.
(year is a kind of period of time)
'in gallery earlier' is the wordplay.
'in' indicates the answer is hidden within the clue.
'YEAR' is hidden within 'gallery earlier'.
'presented' is the link.
(Other definitions for year that I've seen before include "Time period" , "See 19" , "One-tenth of a decade" , "1961?" , "See 33 Across" .)