Former petty officer's exhibitions (5)
I believe the answer is:
expos
'exhibitions' is the definition.
('expo' can be a synonym of 'exhibition')
'former petty officer's' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'petty officer's' becomes 'po's' (naval abbreviation).
'ex'+'pos'='EXPOS'
(Other definitions for expos that I've seen before include "International exhibitions" , "International shows" , "Public displays" .)