Sub is on time to put up fight (6)
I believe the answer is:
resist
'up fight' is the definition.
'resist' can be an answer for 'fight' (resisting is a kind of fighting). I am not certain of the 'up' bit.
'sub is on time' is the wordplay.
'sub' becomes 'res' (I am not sure about this - if you are sure you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'res'+'is'+'t'='RESIST'
'to put' acts as a link.
Can you help me to learn more?
(Other definitions for resist that I've seen before include "Endure" , "Withstand, struggle against" , "Stand up to, fight against" , "Stand firm against" , "Fight back" .)