Certainly not a Tory boy, soldier possibly (9)
I believe the answer is:
consonant
'certainly not a' is the definition.
(the letter A is a vowel, not a consonant)
'tory boy soldier possibly' is the wordplay.
'tory' becomes 'con' (abbreviation for Conservative).
'boy' becomes 'son' (synonyms).
'soldier possibly' becomes 'ant' (as in army ants).
'con'+'son'+'ant'='CONSONANT'
(Other definitions for consonant that I've seen before include "Letter; harmonious chord" , "not I, nor you, one hears" , "B, C, D or F, perhaps?" , "One of many letters" , "In agreement" .)