Earnest European getting behind how metric system counts (7)
I believe the answer is:
intense
'earnest' is the definition.
(thesaurus)
'european getting behind how metric system counts' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'getting behind' says to put letters next to each other (some letters go behind others).
'how metric system counts' becomes 'in tens'.
'e' put after 'intens' is 'INTENSE'.
(Other definitions for intense that I've seen before include "Powerful" , "Great" , "Extreme - strong" , "In an extreme degree" , "Highly emotional" .)