During service, the setter's torn apart (5)
I believe the answer is:
riven
'torn apart' is the definition.
(I've seen this before)
'during service the setter's' is the wordplay.
'during' is an insertion indicator.
'service' becomes 'RN' (abbreviation for Royal Navy).
'the setter's' becomes 'ive' ('I've', referring to the crossword setter).
'rn' enclosing 'ive' is 'RIVEN'.
(Other definitions for riven that I've seen before include "Being cracked" , "Torn apart" , "Split" .)