One that’s fostered trouble with power and support (7)
I believe the answer is:
adoptee
'one that's fostered' is the definition.
(someone adopted or fostered)
'trouble with power and support' is the wordplay.
'trouble' becomes 'ado' (both can mean bother or commotion).
'with' means one lot of letters go next to another.
'power' becomes 'p'.
'and' says to put letters next to each other.
'support' becomes 'tee' (tee is a kind of support).
'ado'+'p'+'tee'='ADOPTEE'
(Other definitions for adoptee that I've seen before include "One taken on by new parents" , "one who's left home" , "one's taken into the family" , "Child taken on by another family" .)