Coward: man ultimately behind dreadfully poor lot (8)
I believe the answer is:
poltroon
'coward' is the definition.
(poltroon is a kind of coward)
'man ultimately behind dreadfully poor lot' is the wordplay.
'ultimately' says to take the final letters.
'behind' says to put letters next to each other.
'dreadfully' indicates an anagram.
The last letter of 'man' is 'n'.
'poor'+'lot'='poorlot'
'poorlot' anagrammed gives 'poltroo'.
'n' put after 'poltroo' is 'POLTROON'.
(Other definitions for poltroon that I've seen before include "one was entirely yellow" , "Traditional chicken" , "Utter coward" , "Abject coward" .)