Trouble returns in so stormy a relationship (7)
I believe the answer is:
liaison
'a relationship' is the definition.
(liaison is a kind of relationship)
'trouble returns in so stormy' is the wordplay.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
'returns' is a reversal indicator.
'stormy' is an anagram indicator.
'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 "Illicit romance" , "Intimate relationship" , "Mediator" , "A contact or affair" , "entanglement" .)