Very ardent about length (4)
I believe the answer is:
yard
'length' is the definition.
(I have seen 'A length ' mean 'yard' so perhaps 'length' could also mean 'yard')
'very ardent about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'YARD' is hidden within 'very ardent'.
(Other definitions for yard that I've seen before include "American garden" , "Three feet U patio" , "3 feet" , "Unit of distance" , "Dray (anag.)" .)