Small men? Ultimately hopeless mates (6)
I believe the answer is:
screws
'mates' is the definition.
(screwing is a kind of mating)
'small men? ultimately hopeless' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'men?' becomes 'crew' (I've seen this in another clue).
'ultimately' says to take the final letters.
The last letter of 'hopeless' is 's'.
's'+'crew'+'s'='SCREWS'
(Other definitions for screws that I've seen before include "Prisoner guards, colloquially" , "Put on" , "Grooved type of nails" , "Fix in place (6)" , "Fasteners with threaded shanks" .)