Very arduous, holding 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 arduous holding' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'YARD' can be found hidden inside 'very arduous'.
(Other definitions for yard that I've seen before include "Three feet - enclosed area" , "area behind the house" , "Thirty six inches" , "Enclosed area; length" , "American garden" .)