Time those people start to understand getting covered in sweet sauce to make a favourable impression (3,3,7)
I believe the answer is:
cut the mustard
'make a favourable impression' is the definition.
(I've seen this before)
'time those people start to understand getting covered in sweet sauce' is the wordplay.
'time' becomes 't' (abbreviation).
'those people' becomes 'them' ('they' refers to a group of people).
'start to' suggests taking the first letters.
'getting covered in' is an insertion indicator.
'sweet sauce' becomes 'custard' (I've seen this before).
The first letter of 'understand' is 'u'.
't'+'them'+'u'='tthemu'
'tthemu' put into 'custard' is 'CUT THE MUSTARD'.
'to' acts as a link.
(Other definitions for cut the mustard that I've seen before include "Perform to standard" , "Reach a required standard" , "should prove acceptable" , "Make the grade" , "do well enough" .)