Clash pressing sweetheart about sink (8)
I believe the answer is:
disagree
'clash' is the definition.
(thesaurus)
'pressing sweetheart about sink' is the wordplay.
'pressing' becomes 'dire' (both can mean urgent).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'about' means one lot of letters goes inside another.
'sink' becomes 'sag' (sagging is a kind of sinking**).
'dire'+'e'='diree'
'diree' placed around 'sag' is 'DISAGREE'.
(Other definitions for disagree that I've seen before include "Beg to differ" , "Hold different opinions" , "Differ in opinion" , "Have differing opinions" , "Dire ages (anag.)" .)