Self-assertive bachelor lacking prudence (5)
I believe the answer is:
brash
'self-assertive' is the definition.
(thesaurus)
'bachelor lacking prudence' is the wordplay.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'lacking prudence' becomes 'rash' (I am not sure about this - if you are sure you should believe this answer much more).
'b'+'rash'='BRASH'
Can you help me to learn more?
(Other definitions for brash that I've seen before include "Cocksure" , "Foolhardy" , "Pushy" , "loose broken rock" , "Loud and rude" .)