Ducks cook gently when lid's removed, son sees occasionally (8)

'ducks' is the definition.
(to duck is to immerse into liquid)

'cook gently when lid's removed son sees occasionally' is the wordplay.
'cook gently' becomes 'simmer' (I've seen this before).
'when lid's removed' means to remove the first letter.
'son' becomes 's' (genealogical abbreviation for son).
'occasionally' means one should take alternating letters.
The alternating letters of 'sees' are 'es'.
'simmer' with its initial letter taken off is 'immer'.
'immer'+'s'+'es'='IMMERSES'