Huge one seen running after Frenchmen (7)
I believe the answer is:
immense
'huge' is the definition.
(I know that huge can be written as immense)
'one seen running after frenchmen' is the wordplay.
'one' becomes 'i' (Roman numeral).
'running' is an anagram indicator (the letters run around to different places).
'after' says to put letters next to each other.
'frenchmen' becomes 'MM' (two M's - M is short for Monsieur).
'seen' is an anagram of 'ense'.
'ense' after 'mm' is 'mmense'.
'i'+'mmense'='IMMENSE'
(Other definitions for immense that I've seen before include "'Gigantic, extensive (7)'" , "Very good" , "Unctuously ingratiating" , "Jumbo" , "Vast or enormous" .)