Insect starts to migrate over the heath (4)
I believe the answer is:
moth
'insect' is the definition.
(moth is a kind of insect)
'starts to migrate over the heath' is the wordplay.
'starts to' indicates taking the first letters.
The initial letters of 'migrate over the heath' is 'MOTH'.
(Other definitions for moth that I've seen before include "Insect drawn to flame" , "Fairy like 13" , "Nocturnal winged insect" , "one drawn to the bright lights" , "Shakespearean fairy?" .)