A good fellow, in love, will get busy (4,5)
I believe the answer is:
look alive
'get busy' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'a good fellow in love' is the wordplay.
'a good' becomes 'ok'.
'fellow' becomes 'ali' (man's name).
'in' indicates putting letters inside.
'ok'+'ali'='okali'
'okali' inserted into 'love' is 'LOOK ALIVE'.
'will' is the link.
Can you help me to learn more?
(Another definition for look alive that I've seen is " Come on".)