Four unfinished, strange, small craft (4)
I believe the answer is:
ufos
'craft' is the definition.
(I have seen 'Alien craft ' mean 'ufos' so perhaps 'craft' could also mean 'ufos')
'four unfinished strange small' is the wordplay.
'unfinished' means to remove the last letter.
'strange' indicates anagramming the letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'four' with its last letter taken away is 'fou'.
'fou' with letters rearranged gives 'ufo'.
'ufo'+'s'='UFOS'
(Other definitions for ufos that I've seen before include "Initially and formally, flying saucers" , "Initially, official flying saucers" , "Roswell sightings?" , "Supposed spaceships" , "Strange aerial objects" .)