Dancing, a girl hadn't to lose all momentum (5,2,1,4)
I believe the answer is:
grind to a halt
'lose all momentum' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'dancing a girl hadn't to' is the wordplay.
'dancing' is an anagram indicator.
'a'+'girl'+'hadnt'+'to'='agirlhadntto'
'agirlhadntto' is an anagram of 'GRIND TO A HALT'.
Can you help me to learn more?
(Other definitions for grind to a halt that I've seen before include "Gradually cease moving" , "Come to a standstill" , "Slowly become stationary" , "Become unable to move any further" , "Stop gradually but finally" .)