Fancy the prom with sea air (10)
I believe the answer is:
atmosphere
'air' is the definition.
('atmosphere' can be a synonym of 'air')
'fancy the prom with sea' is the wordplay.
'fancy' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'the'+'prom'='theprom'
'theprom' put after 'sea' is 'seatheprom'.
'seatheprom' with letters rearranged gives 'ATMOSPHERE'.
(Other definitions for atmosphere that I've seen before include "Spirit" , "Air" , "Feeling" , "We are always enveloped in this" , "character" .)