Escaping deranged hunter's trailing leg (2,3,3)
I believe the answer is:
on the run
'escaping' is the definition.
(on the run means in the process of escaping)
'deranged hunter's trailing leg' is the wordplay.
'deranged' indicates anagramming the letters.
'trailing' means one lot of letters go next to another.
'leg' becomes 'on' (cricket term, fielding positions).
'hunter' anagrammed gives 'therun'.
'therun' put after 'on' is 'ON THE RUN'.
(Other definitions for on the run that I've seen before include "In flight as from the police" , "Marathon contestants are" , "one after the other" , "Evading capture" , "Fleeing arrest" .)