A train breaks down after five -- that's different! (7)
I believe the answer is:
variant
'different' is the definition.
('variant' can be similar in meaning to 'different')
'a train breaks down after five' is the wordplay.
'breaks down' is an anagram indicator.
'after' means one lot of letters go next to another.
'five' becomes 'v' (roman numeral).
'train' anagrammed gives 'riant'.
'a'+'riant'='ariant'
'ariant' put after 'v' is 'VARIANT'.
'that's' is the link.
(Other definitions for variant that I've seen before include "Form, version" , "Alternative" , "Non-standard version" , "Differing from the norm" , "different from the norm" .)