Working by sight is exactly right (4,2)
I believe the answer is:
spot on
'exactly right' is the definition.
(I've seen this before)
'working by sight' is the wordplay.
'working' becomes 'on'.
'by' is a charade indicator (letters next to each other).
'sight' becomes 'spot' (spotting is a kind of sighting**).
'on' put after 'spot' is 'SPOT-ON'.
'is' is the link.
(Other definitions for spot on that I've seen before include "Correct" , "What the leading actor may have" , "what soiled garment had?" , "Exactly right" , "Quite right" .)