The Romans imagined him to be two-faced (5)
I believe the answer is:
janus
'the romans imagined him to be two-faced' is the definition.
The definition suggests a singular noun which matches the answer.
'him to be two-faced' is the wordplay.
'him' becomes 'Jan'.
'to be two-faced' becomes 'us' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'jan'+'us'='JANUS'
Can you help me to learn more?
(Other definitions for janus that I've seen before include "Two-faced Roman god of doors (5)" , "Two-faced god of gates and doors - Guardian cryptic crossword setter" , "Roman deity who faces front and back" , "Two-faced Roman god who guarded doors and gates" , "Two-faced person" .)