Merge popular expression with one unknown (8)
I believe the answer is:
intermix
'merge' is the definition.
(thesaurus)
'popular expression with one unknown' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'expression' becomes 'term' (both can mean a piece of vocabulary).
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'unknown' becomes 'x' (algebra).
'in'+'term'+'i'+'x'='INTERMIX'
(Other definitions for intermix that I've seen before include "Stir" , "Blend together" , "Mingle" .)