Gradually introduce argument during preliminaries (3,2)
I believe the answer is:
run in
'gradually introduce' is the definition.
(as in to begin using something slowly)
'argument during preliminaries' is the wordplay.
I cannot really understand how this works, but
'during' could be 'in' (I've seen this in other clues) and 'in' is located in the answer.
The remaining letters 'run' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for run in that I've seen before include "Disagreement (with authority?)" , "Carefully bring to full working order - arrest" , "Dispute" , "Altercation" , "Quarrel; set-to" .)