Examine small tin (4)
I believe the answer is:
scan
'examine' is the definition.
(scanning is a kind of examining)
'small tin' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tin' becomes 'can' (synonyms).
's'+'can'='SCAN'
(Other definitions for scan that I've seen before include "MRI or CAT say" , "Perusal" , "Survey - fit in metrically" , "Survey, maybe by X-ray" , "Analyse rhythm of line of verse" .)