Man on the lookout when old boy leaves (7)
I believe the answer is:
servant
'man' is the definition.
(thesaurus)
'the lookout when old boy leaves' is the wordplay.
I cannot really see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'boy' could be 'van' and 'van' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'on' acts as a link.
Can you help me to learn more?
(Other definitions for servant that I've seen before include "Worker in household" , "Maid or butler, say" , "Housekeeper" , "House employee" , "eg Sancho Panza" .)