Think to charge a quid (4)
I believe the answer is:
feel
'think' is the definition.
(feeling is a kind of thinking)
'charge a quid' is the wordplay.
'charge' becomes 'fee' (fee is a kind of charge).
'a quid' becomes 'l' (I can't explain this - if you can you should believe this answer much more).
'fee'+'l'='FEEL'
'to' is the link.
Can you help me to learn more?
(Other definitions for feel that I've seen before include "impression" , "Find by groping" , "Have sensation of" , "Instinct" , "Experience, perceive" .)