Is standing right by the gentleman (3)
I believe the answer is:
sir
'the gentleman' is the definition.
(I've seen this before)
'is standing right by' is the wordplay.
'is standing' becomes 'si' (I've seen this in other clues).
'right' becomes 'r' (common abbreviation).
'by' is a charade indicator (letters next to each other).
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Way to address gentleman" , "Formal and deferential address to a man" , "Noble title of undesirable kind" , "Formal term of address" , "Respectful address for a man" .)