Man tapped on the shoulder is turning back to customer (3)
I believe the answer is:
sir
'man' is the definition.
(sir is a kind of man)
'is turning back to customer' is the wordplay.
'turning' is a reversal indicator.
'back to' suggests the final letters.
The last letter of 'customer' is 'r'.
'is' reversed gives 'si'.
'si'+'r'='SIR'
'tapped on the shoulder' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Title of knight or baronet" , "Form of address for a knight" , "Poet and novelist" , "Formal and deferential address to a man" , "address to teacher" .)