The first of the hydrogen weapons causes damage (5)
I believe the answer is:
harms
'causes damage' is the definition.
Although both the answer and definition are verbs in their -s form, I can't understand how one could define the other.
'the first of the hydrogen weapons' is the wordplay.
'the first of' indicates taking the first letters.
'weapons' becomes 'arms' ('arm' can be a synonym of 'weapon').
The first letter of 'hydrogen' is 'h'.
'h'+'arms'='HARMS'
Can you help me to learn more?
(Other definitions for harms that I've seen before include "Hurts, damages" , "Wounds" , "Injures, damages" .)