Language of lovers evolves (4)
I believe the answer is:
erse
'language' is the definition.
(historical term for Scottish Gaelic)
'of lovers evolves' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ERSE' is hidden within 'lovers evolves'.
(Other definitions for erse that I've seen before include "Old Celtic language" , "Irish Gaelic language" , "Kind of Gaelic for seer" , "Old Gaelic for the seer" , "Old language spoken by strange seer" .)