Courier on steed heads off when they give their approval (7)
I believe the answer is:
endorse
'approval' is the definition.
(I have seen 'Express approval' mean 'endorse' so perhaps 'approval' could also mean 'endorse')
'courier on steed heads off' is the wordplay.
'courier on' becomes 'end' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'steed' becomes 'horse ' (I've seen this in another clue).
'heads off' means to remove the first letter.
'horse' with its first letter taken away is 'orse'.
'end'+'orse'='ENDORSE'
'when they give their' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for endorse that I've seen before include "Favour" , "Pass" , "Back, sanction" , "Give backing to" , "Say you approve" .)