Challenge one heartless man confining a dog (6)
I believe the answer is:
impugn
'challenge' is the definition.
(impugning is a kind of challenging)
'one heartless man confining a dog' is the wordplay.
'one' becomes 'i' (Roman numeral).
'heartless' says to hollow out the word (remove centre letters).
'confining' is an insertion indicator.
'a dog' becomes 'pug' (pug is a kind of dog).
'man' with its centre taken out is 'mn'.
'mn' placed around 'pug' is 'mpugn'.
'i'+'mpugn'='IMPUGN'
(Other definitions for impugn that I've seen before include "Express doubts about truth or honesty of, say motives" , "Attack verbally" , "Blacken the name of" , "Nip mug badly and call in question" , "Attack as false, challenge" .)