Monster caught breaking partner's back (6)
I believe the answer is:
scylla
'monster' is the definition.
(I've seen this before)
'caught breaking partner's back' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'breaking' indicates putting letters inside.
'partner's' becomes 'ally's' (an ally is an associate or supporter).
'back' says the letters should be written backwards.
'c' inserted into 'allys' is 'allycs'.
'allycs' in reverse letter order is 'SCYLLA'.
(Other definitions for scylla that I've seen before include "Six-headed monster who sat on a rock opposite Charybdis" , "Female sea monster in Greek myth, lived opposite whirlpool Charybdis" , "legendary menace in Med" , "Mythic sea nymph transformed into a sea monster who devoured sailors" , "danger to shipping" .)