Meet up and force argument (4)
I believe the answer is:
tiff
'argument' is the definition.
(thesaurus)
'meet up and force' is the wordplay.
'meet' becomes 'fit' (fitting is a kind of meeting).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'and' says to put letters next to each other.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'fit' in reverse letter order is 'tif'.
'tif'+'f'='TIFF'
(Other definitions for tiff that I've seen before include "Row; argument" , "Hard to lose opening argument" , "A spat or small row" , "Petty argument" , "Minor squabble" .)