Mocking one with small chest (7)
I believe the answer is:
scoffer
'mocking one' is the definition.
(I've seen this in another clue)
'small chest' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'chest' becomes 'coffer' (coffer is a kind of chest).
's'+'coffer'='SCOFFER'
'with' acts as a link.
(Other definitions for scoffer that I've seen before include "Person deriding" , "One who scorns" , "I doubt" , "One derides" , "Scornfully derisive person" .)