Strike up, as usual (3)
I believe the answer is:
par
'usual' is the definition.
'strike up' is the wordplay.
'strike' becomes 'rap' (rapping is a kind of striking).
'up' is a reversal indicator (in down clue: letters go upwards).
'rap' in reverse letter order is 'PAR'.
'as' is the link.
(Other definitions for par that I've seen before include "Normal number of strokes for a hole at golf" , "Standard score for golfers" , "Mean" , "Rap to get even" , "Normal golfing score" .)