Possibly too plain, it's not compulsory (8)
I believe the answer is:
optional
'not compulsory' is the definition.
(I've seen this before)
'possibly too plain' is the wordplay.
'possibly' indicates an anagram.
'too'+'plain'='tooplain'
'tooplain' is an anagram of 'OPTIONAL'.
'it's' acts as a link.
(Other definitions for optional that I've seen before include "Open to choice - take your pick" , "Platoon I join as a matter of choice" , "Voluntary - not required" , "Such as one can take or leave" , "By choice, elective" .)