Unusual, partly for a reason (4)
I believe the answer is:
rare
'unusual' is the definition.
(rare can mean unusual or odd)
'partly for a reason' is the wordplay.
'partly' indicates the answer is hidden within the clue (part of the clue is used).
'RARE' is hidden in the letters of 'for a reason'.
(Other definitions for rare that I've seen before include "Might a four-leaf clover be so" , "little done?" , "Singular, infrequent" , "Seldom seen or met" , "Not often seen - underdone" .)