Line up, like soldiers with foot problem quarrelling (7,3)
I believe the answer is:
falling out
'quarrelling' is the definition.
(falling out is a kind of quarreling)
'line up like soldiers with foot problem' is the wordplay.
'line up like soldiers' becomes 'fall in' (fall in can mean to line up in formation).
'with' is a charade indicator (letters next to each other).
'foot problem' becomes 'gout' (disease affecting feet bones).
'fallin'+'gout'='FALLING OUT'
(Other definitions for falling out that I've seen before include "leaving parade ground" , "leaving by the window?" , "Breaking ranks" , "Row" , "Quarrelling" .)