Sudden move to the centre in close-pressed crowd losing its head (6)
I believe the answer is:
inrush
'sudden move to the centre' is the definition.
'in close-pressed crowd losing its head' is the wordplay.
'close-pressed crowd' becomes 'crush'.
'losing its head' means to remove the first letter.
'crush' with its first letter taken off is 'rush'.
'in'+'rush'='INRUSH'
(Other definitions for inrush that I've seen before include "Sudden move inwards" , "Sudden inward flow" , "Sudden arrival (of people or water?)" , "Overwhelming flow" , "hasty entrance" .)