Particular start of sequence optional (9)
I believe the answer is:
selective
'particular' is the definition.
(thesaurus)
'start of sequence optional' is the wordplay.
'start of' suggests taking the first letters.
'optional' becomes 'elective' ('elective' can be similar in meaning to 'optional').
The initial letter of 'sequence' is 's'.
's'+'elective'='SELECTIVE'
(Other definitions for selective that I've seen before include "Tending to choose carefully" , "Choosing carefully" , "Choosy" .)