Watch old boy attend to customers (7)
I believe the answer is:
observe
'watch old' is the definition.
'observe' can be an answer for 'watch' (observing is a kind of watching). I am not certain of the 'old' bit.
'attend to customers' is the wordplay.
'attend' becomes 'serve' (synonyms).
'to' means one lot of letters go next to another (I've seen this in other clues).
'customers' becomes 'ob' (I can't justify this - if you can you should believe this answer much more).
'serve' after 'ob' is 'OBSERVE'.
'boy' is the link.
Can you help me to learn more?
(Other definitions for observe that I've seen before include "Notice, watch" , "Watch attentively" , "Espy" , "See - keep - remark - maintain - celebrate" , "Discern" .)