Monstrous female and two sons therefore turned back (6)
I believe the answer is:
ogress
'monstrous female' is the definition.
(I've seen this before)
'two sons therefore turned back' is the wordplay.
'two sons' means two substitutions for 'sons'.
'sons' becomes 's'.
'sons' becomes 's'.
'therefore' becomes 'ergo' (I've seen this before).
'turned back' is a reversal indicator.
's'+'s'+'ergo'='ssergo'
'ssergo' reversed gives 'OGRESS'.
'and' acts as a link.
(Other definitions for ogress that I've seen before include "Terrifying female" , "Some progressive" , "Monstrous woman" , "'bloody difficult woman' ... ?" , "Man-eating giantess" .)