The action of making use of boxer starting off on bit of filming (6)
I believe the answer is:
uptake
'the action' is the definition.
(uptake is a kind of action)
'of filming' is the wordplay.
'of' becomes 'up' (I can't explain this - if you can you should give a lot more credence to this answer).
'filming' becomes 'take' (take is a kind of filming).
'up'+'take'='UPTAKE'
'of making use of boxer starting off on bit' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for uptake that I've seen before include "Grasp one can be quick on" , "Put off until later" , "Ability to understand what is meant - some are quick on it" , "Comprehension" , "Ability to understand what is meant - some are slow on it" .)