A boxer good at putting up a guard? (8)
I believe the answer is:
watchdog
'a guard?' is the definition.
'a boxer good at putting up' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'boxer' could be 'dog' (I've seen this in other clues) and 'dog' is found in the answer.
This explanation may well be incorrect...
(Other definitions for watchdog that I've seen before include "Animal guarding property - guardian against inefficiency or illegality" , "TV consumer programme" , "Heroic poem" , "Guardian with teeth?" , "Ombudsman" .)