Put out by merely half done service (5)
I believe the answer is:
douse
'put out' is the definition.
(as in to douse a fire)
'half done service' is the wordplay.
'half' indicates taking half.
'service' becomes 'use' (to be of service is to be of use).
'done' halved is 'do'.
'do'+'use'='DOUSE'
'by merely' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for douse that I've seen before include "Throw water over, extinguish" , "Put out (flame)" , "Plunge into water" , "Soak" , "snuff" .)