Vacant school needing cardinal to show manual dexterity (7)
I believe the answer is:
sleight
'manual dexterity' is the definition.
('sleight' can be a synonym of 'manual dexterity')
'vacant school needing cardinal' is the wordplay.
'vacant' means to remove the middle letters.
'needing' says to put letters next to each other.
'cardinal' becomes 'eight' ('eight' can be similar in meaning to 'cardinal'**).
'school' with its middle taken out is 'sl'.
'sl'+'eight'='SLEIGHT'
'to show' acts as a link.
(Other definitions for sleight that I've seen before include "Hits leg (anag) - cunning" , "It's deceiving to the eye" , "Handy skill" , "craft" , "Cunning action" .)