One insect hiding in edges of tussocky grass (7)
I believe the answer is:
timothy
'grass' is the definition.
(I know that timothy is a type of grass)
'one insect hiding in edges of tussocky' is the wordplay.
'one' becomes 'i' (Roman numeral).
'insect' becomes 'moth' (moth is a kind of insect).
'hiding in' is an insertion indicator.
'edges of' suggests removing the centre.
'tussocky' with its centre taken out is 'ty'.
'i'+'moth'='imoth'
'imoth' inserted inside 'ty' is 'TIMOTHY'.
(Other definitions for timothy that I've seen before include "A grass; an NT disciple" , "haymaker?" , "Grass; boy's name" , "New Testament book" , "Letter recipient" .)