The awful pair returned, determined on becoming thieves (7)
I believe the answer is:
pirates
'thieves' is the definition.
(pirating is a kind of thieving)
'the awful pair returned determined on' is the wordplay.
'the awful' is an anagram indicator.
'returned' says the letters should be written backwards.
'determined' becomes 'set' (synonyms).
'on' means one lot of letters go next to another.
'set' in reverse letter order is 'tes'.
'pair' anagrammed gives 'pira'.
'pira'+'tes'='PIRATES'
'becoming' is the link.
(Other definitions for pirates that I've seen before include "Robbers at sea" , "Sea trip (anag.)" , "Chatters, talks foolishly" , "Bad sailors" , "felons found in craft?" .)