Former lover with old sign of nerves? That's unusual (6)
I believe the answer is:
exotic
'unusual' is the definition.
(I've seen this before)
'former lover with old sign of nerves?' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'with' is a charade indicator (letters next to each other).
'old' becomes 'o' (abbreviation).
'sign of nerves?' becomes 'tic' (as in a nervous tic).
'ex'+'o'+'tic'='EXOTIC'
'that's' is the link.
(Other definitions for exotic that I've seen before include "Strikingly strange or unusual" , "Alluringly foreign" , "Foreign; strikingly colourful" , "Glamorous or strange from abroad" , "Having a strange beauty" .)