Step inside, sir, thank you (5)
I believe the answer is:
stair
'step' is the definition.
(a single step in a staircase)
'inside sir thank you' is the wordplay.
'inside' is an insertion indicator.
'thank you' becomes 'ta' (informal way of saying thank you).
'sir' enclosing 'ta' is 'STAIR'.
(Other definitions for stair that I've seen before include "Support on which to step" , "Step in house" , "something of a case" , "single step in a series" , "One step in a series" .)