Spirit initially expresses life force (5)
I believe the answer is:
impel
'force' is the definition.
(I know that force can be written as impel)
'spirit initially expresses life' is the wordplay.
'spirit' becomes 'imp' (I have seen 'Evil spirit ' mean 'imp' so perhaps 'spirit' could also mean 'imp').
'initially' suggests taking the first letters.
The initial letters of 'expresses life' is 'el'.
'imp'+'el'='IMPEL'
(Other definitions for impel that I've seen before include "Spur on" , "'Drive, force (5)'" , "Oblige" , "Force into motion" , "Send or drive forward" .)