He is not moving - how amusing is that? (5,3)
I believe the answer is:
inert gas
'he' is the definition.
(symbol for Helium)
'not moving how amusing is that?' is the wordplay.
I cannot quite see how this works, but
'not moving' could be 'inert' (I've seen this before) and 'inert' is found in the answer.
The remaining letters 'gas' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for inert gas that I've seen before include "He perhaps" , "provokes no reaction" , "One expects no reaction from such" , "Eg, helium or argon" , "Eg, krypton or argon" .)