Showing mercy, releasing one in vault (6)
I believe the answer is:
spring
'vault' is the definition.
(both can mean to jump)
'showing mercy releasing one' is the wordplay.
'showing mercy' becomes 'sparing' (I've seen this in another clue).
'releasing' suggests deleting specific letters.
'one' becomes 'a' (a thing is one thing).
'sparing' with 'a' taken out is 'SPRING'.
'in' is the link.
(Other definitions for spring that I've seen before include "Shoot forth" , "Leap - opposite to 15" , "Well - bound" , "May, perhaps - bound" , "Spa" .)