Not starting trouble - that's different (5)
I believe the answer is:
other
'different' is the definition.
(similar in meaning)
'not starting trouble' is the wordplay.
'not starting' means to remove the first letter.
'trouble' becomes 'bother' (synonyms).
'bother' with its initial letter taken off is 'OTHER'.
'that's' acts as a link.
(Other definitions for other that I've seen before include "Distinct" , "Different; alternate" , "Alternative - unrelated" , "not like you" , "Alternate; additional" .)