Legally seize girl's coach (8)
I believe the answer is:
distrain
'legally seize' is the definition.
(I've seen this before)
'girl's coach' is the wordplay.
'girl's' becomes 'Dis' (short for Diane or Diana).
'coach' becomes 'train' (both can mean to instruct a sportsperson).
'dis'+'train'='DISTRAIN'
(Other definitions for distrain that I've seen before include "Legally seize (property)" , "... Requisition" , "Seize (property) to recover money owed (Law)" , "Seize possessions as payment for debt" , "Seize goods legally" .)