Tell, say, when demonstrator loses his head (6)
I believe the answer is:
archer
'tell say' is the definition.
'archer' can be an answer for 'tell' (I've seen this before). I am not certain of the 'say' bit.
'demonstrator loses his head' is the wordplay.
'demonstrator' becomes 'marcher' (I've seen this before).
'loses his head' means to remove the first letter.
'marcher' with its initial letter taken off is 'ARCHER'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for archer that I've seen before include "One uses a bow and arrow" , "Jeffrey, bowman?" , "Toxophilite" , "Bow and arrow user" , "Tell possibly < more mischievous" .)