Examine small tin (4)
'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).
(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".)