Help against area in trouble (5)
I believe the answer is:
avail
'help' is the definition.
('avail' can be a synonym of 'help')
'against area in trouble' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus'**).
'area' becomes 'a' (maths abbreviation).
'in' indicates putting letters inside.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'v'+'a'='va'
'va' placed into 'ail' is 'AVAIL'.
(Other definitions for avail that I've seen before include "(Of no) use or benefit" , "Serve a purpose" , "Be useful" , "Help, benefit" , "See 3" .)