For centuries it was the language of the masses (5)
I believe the answer is:
latin
'the language of the masses' is the definition.
(I've seen this in another clue)
'for centuries it' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'lain' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'was' is the link.
Can you help me to learn more?
(Other definitions for latin that I've seen before include "The language of Virgil" , "Virgil's tongue" , "Ancestor of French, Romanian" , "ancient tongue" , "Parent language of French,Romanian" .)