Loose coins ruin her plans (8)
I believe the answer is:
shrapnel
'loose coins' is the definition.
(shrapnel can mean spare change)
'ruin her plans' is the wordplay.
'ruin' indicates anagramming the letters.
'her'+'plans'='herplans'
'herplans' with letters rearranged gives 'SHRAPNEL'.
(Other definitions for shrapnel that I've seen before include "Scattered pieces from a bursting shell" , "Colloquially, small change" , "Metal bomb fragments" , "Pieces scattered after a shell explodes" , "Exploding shell fragments" .)