Progress with play, mostly sardonic (7)
I believe the answer is:
inroads
'progress' is the definition.
(I've seen this before)
'play mostly sardonic' is the wordplay.
'play' indicates an anagram.
'mostly' means to remove the last letter.
'sardonic' with its last letter taken away is 'sardoni'.
'sardoni' anagrammed gives 'INROADS'.
'with' acts as a link.
(Other definitions for inroads that I've seen before include "Sad iron (anag.)" , "Encroachments" , "Major acts of encroachment" , "large quantities consumed when these are made" , "Progress" .)