Exploit one of those losing the head at The Post on edge (4,9)
I believe the answer is:
take advantage
'exploit' is the definition.
(I've seen this before)
'at the post on edge' is the wordplay.
'at' means one lot of letters go next to another.
'the post on' becomes 'take' (I am not sure about this - if you are sure you should believe this answer much more).
'edge' becomes 'advantage' (I've seen this before).
'take' next to 'advantage' is 'TAKE ADVANTAGE'.
'one of those losing the head' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Another definition for take advantage that I've seen is " Exploit".)