Listening device that is intercepted by line near the start (5,2)
I believe the answer is:
early on
'near the start' is the definition.
(I've seen this in another clue)
'listening device that is intercepted by line' is the wordplay.
'listening device' becomes 'ear' (I've seen this in another clue).
'that' becomes 'yon' (archaic term meaning 'that').
'is intercepted by' is an insertion indicator (I've seen this in other clues).
'line' becomes 'l' (used when specifying particular lines from a poem).
'ear'+'yon'='earyon'
'earyon' placed around 'l' is 'EARLY ON'.
(Other definitions for early on that I've seen before include "Towards the beginning" , "Previously" , "In the first stage" , "Before much time has elapsed" , "Quite soon in the proceedings" .)