Back trouble in so stormy an affair (7)
I believe the answer is:
liaison
'an affair' is the definition.
('liaison' can be a synonym of 'affair')
'back trouble in so stormy' is the wordplay.
'back' is a reversal indicator.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
'stormy' indicates anagramming the letters.
'ail' reversed gives 'lia'.
'in'+'so'='inso'
'inso' is an anagram of 'ison'.
'lia'+'ison'='LIAISON'
(Other definitions for liaison that I've seen before include "Secretive affair" , "Naughty affair?" , "Illicit romance" , "Go-between" , "Contact between groups" .)