That man with nuclear particles is extremely difficult (9)
I believe the answer is:
herculean
'extremely difficult' is the definition.
'that man with nuclear particles' is the wordplay.
'that man' becomes 'he'.
'with' is a charade indicator (letters next to each other).
'particles' indicates an anagram (the same parts or particles in a new order).
'nuclear' with letters rearranged gives 'rculean'.
'he'+'rculean'='HERCULEAN'
'is' is the link.
(Other definitions for herculean that I've seen before include "Colossal" , "Old Labour" , "Requiring very great effort" , "So enormous a task" , "Showing great strength" .)