Papa constrained by fixed idea, notion being dogmatic (11)
I believe the answer is:
opinionated
'dogmatic' is the definition.
(thesaurus)
'papa constrained by fixed idea notion' is the wordplay.
'papa' becomes 'p' (phonetic alphabet: alpha, bravo, charlie etc.).
'constrained by' means one lot of letters goes inside another.
'fixed' is an anagram indicator.
'idea'+'notion'='ideanotion'
'ideanotion' anagrammed gives 'oinionated'.
'p' placed within 'oinionated' is 'OPINIONATED'.
'being' acts as a link.
(Other definitions for opinionated that I've seen before include "Assertive in one's views" , "Doctrinaire" , "Assertively dogmatic" .)