Superiority in mother dear's stifling? (7)
I believe the answer is:
primacy
'superiority' is the definition.
(thesaurus)
'mother dear's stifling?' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'dear' becomes 'pricy' (synonyms).
'stifling?' is an insertion indicator (some letters get stifled or contained).
'ma' going within 'pricy' is 'PRIMACY'.
'in' acts as a link.
(Other definitions for primacy that I've seen before include "Dominance" , "precedence" , "first place" .)