A bachelor left monarch more skilful (5)
I believe the answer is:
abler
'more skilful' is the definition.
(I've seen this before)
'a bachelor left monarch' is the wordplay.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'left' becomes 'l' (common abbreviation).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'a'+'b'+'l'+'er'='ABLER'
(Other definitions for abler that I've seen before include "More accomplished" , "More competent, perhaps like seaman" , "stronger" , "More capable" , "More skilful" .)