Tough para’s unusual signature (9)
I believe the answer is:
autograph
'signature' is the definition.
(autograph is a kind of signature)
'tough para's unusual' is the wordplay.
'unusual' indicates anagramming the letters.
'tough'+'para'='toughpara'
'toughpara' anagrammed gives 'AUTOGRAPH'.
(Other definitions for autograph that I've seen before include "A signature given as souvenir" , "a collector's item perhaps" , "Famous person's signature" , "One's signature" , "(Celebrity) signature" .)