Insect starts to multiply on the heath (4)
I believe the answer is:
moth
'insect' is the definition.
(I know that moth is a type of lepidopteran)
'starts to multiply on the heath' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'multiply on the heath' is 'MOTH'.
(Other definitions for moth that I've seen before include "Shakespearean fairy?" , "Usually nocturnal insect" , "Four-winged insect" , "one drawn to the bright lights" , "Insect attracted to light" .)