Unusual play, Act I being modified (8)
I believe the answer is:
atypical
'unusual' is the definition.
(I know that unusual can be written as atypical)
'play act i being modified' is the wordplay.
'being modified' is an anagram indicator.
'play'+'act'+'i'='playacti'
'playacti' anagrammed gives 'ATYPICAL'.
(Other definitions for atypical that I've seen before include "Not conforming to a norm or average" , "Not representative or usual" , "Nonconformist" , "Eccentric" , "Nonconforming" .)