Metropolis is only half the truth (4)
I believe the answer is:
city
'metropolis' is the definition.
(a metropolis is a large city)
'only half the truth' is the wordplay.
'only half' suggests halving the letters.
'the truth' becomes 'veracity' (I've seen this before).
'veracity' halved is 'CITY'.
'is' is the link.
(Other definitions for city that I've seen before include "- of God (St Augustine)" , "Large town" , "Coventry, say" , "it's important to see" , "See 9" .)