They're bound to work out ref's needing specs ultimately (5)
I believe the answer is:
serfs
'they're bound to work' is the definition.
(I've seen this in another clue)
'out ref's needing specs ultimately' is the wordplay.
'out' is an anagram indicator.
'needing' says to put letters next to each other.
'ultimately' indicates one should take the final letters.
The last letter of 'specs' is 's'.
'refs' anagrammed gives 'serf'.
'serf'+'s'='SERFS'
(Other definitions for serfs that I've seen before include "They were bound to work" , "Agricultural labourers tied to estate" , "Feudal labourers" , "Persons owned by a feudal lord of old" , "They were owned by the feudal lord in the Middle Ages" .)