Vitality unknown in Grantchester at first (4)
I believe the answer is:
zing
'vitality' is the definition.
(I've seen this before)
'unknown in grantchester at first' is the wordplay.
'unknown' becomes 'z' (algebra).
'at first' says to take the initial letters.
The initial letter of 'grantchester' is 'g'.
'z'+'in'+'g'='ZING'
(Other definitions for zing that I've seen before include "Thrill" , "Energy - liveliness" , "Energy - high-pitched buzz" , "Go" , "spirit" .)