A high explosive, and yet I'm relaxed (8)
I believe the answer is:
dynamite
'a high explosive' is the definition.
(I've seen this before)
'and yet i'm relaxed' is the wordplay.
'relaxed' indicates anagramming the letters.
'and'+'yet'+'im'='andyetim'
'andyetim' with letters rearranged gives 'DYNAMITE'.
(Other definitions for dynamite that I've seen before include "Jelly" , "Marvellous thing" , "Blast" , "might one be blown away by it?" , "Dainty me could be explosive" .)