Heads of optometry giving lecture explaining eye (4)
I believe the answer is:
ogle
'eye' is the definition.
(I've seen this in another clue)
'heads of optometry giving lecture explaining' is the wordplay.
'heads of' suggests taking the first letters.
The first letters of 'optometry giving lecture explaining' is 'OGLE'.
(Other definitions for ogle that I've seen before include "Look flirtatiously at" , "Eye up lasciviously" , "Leer" , "Give the glad eye" , "Amorous glance" .)