Excitement about a single step (5)
I believe the answer is:
stair
'single step' is the definition.
(I've seen this before)
'excitement about a' is the wordplay.
'excitement' becomes 'stir' (synonyms).
'about' means one lot of letters goes inside another.
'stir' placed around 'a' is 'STAIR'.
(Other definitions for stair that I've seen before include "One of a flight of steps" , "Set of steps" , "A step on the way up or down" , "Step in house" , "Tread and riser" .)