Feller occasionally a parasite (4)
I believe the answer is:
flea
'parasite' is the definition.
(I have seen 'Tiny parasite ' mean 'flea' so perhaps 'parasite' could also mean 'flea')
'feller occasionally a' is the wordplay.
'occasionally' indicates alternate letters.
The alternating letters of 'feller' are 'fle'.
'fle'+'a'='FLEA'
(Other definitions for flea that I've seen before include "In your ear, this insect is a rebuke" , "Small blood-sucking insect" , "Wingless, jumping insect" , "In one's ear, it's a stinging rebuke" , "Blood-sucking parasites" .)