Worry hardly any of them will find this frightening (8)
I believe the answer is:
fearsome
'them will find this frightening' is the definition.
'fearsome' can be an answer for 'frightening' (I've seen this before). I am not sure about the rest of the definition.
'worry hardly any' is the wordplay.
'worry hardly' becomes 'fear' (fearing is a kind of worrying. I am not sure about the 'hardly' bit.).
'any' becomes 'some' (similar in meaning).
'fear'+'some'='FEARSOME'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for fearsome that I've seen before include "Horrendous" , "Terrifying" , "Dreadful" , "Daunting" , "Of frightening appearance" .)