I said I could be on the lookout (3)
I believe the answer is:
eye
'i could be on the lookout' is the definition.
(an eye looks at things)
'i said' is the wordplay.
'said' indicates a 'sounds like' (homophone) clue.
'i' is a homophone of 'EYE'.
(Other definitions for eye that I've seen before include "Body organ" , "Look at with interest" , "May be private" , "'Cast a cold ... on life, on death' (Yeats)" , "Optical organ" .)