To reverse one clasp top on the ear (4)
I believe the answer is:
otic
'the ear' is the definition.
(I've seen this before)
'to reverse one clasp top' is the wordplay.
'reverse' is a reversal indicator.
'one' becomes 'i' (Roman numeral).
'top' says to take the initial letters (in a down clue, the first letter is top).
The initial letter of 'clasp' is 'c'.
'to' written backwards gives 'ot'.
'ot'+'i'+'c'='OTIC'
'on' acts as a link.
(Other definitions for otic that I've seen before include "Listener's" , "organ" , "relating to area incorporating canal" , "about hearing" , "in relation to the listener" .)