Verify a small particular (9)
I believe the answer is:
ascertain
'verify' is the definition.
(ascertaining is a kind of verifying)
'a small particular' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'particular' becomes 'certain' (I've seen this before).
'a'+'s'+'certain'='ASCERTAIN'
(Other definitions for ascertain that I've seen before include "Find out - make sure" , "Establish after an investigation or study" , "Become aware" , "Verify" , "Get to know" .)