Past master with desire to carry on (7)
I believe the answer is:
prolong
'carry on' is the definition.
(prolonging is a kind of carrying on)
'past master with desire' is the wordplay.
'past master' becomes 'pro' (a pro is a master).
'with' says to put letters next to each other.
'desire' becomes 'long' (longing is a kind of desiring).
'pro'+'long'='PROLONG'
'to' acts as a link.
(Other definitions for prolong that I've seen before include "delay" , "Spin out" , "Extend the duration of" , "Extend in length" , "Have extended" .)