Piece of straw is perhaps thus described (4)
I believe the answer is:
wisp
'piece of straw is perhaps thus described' is the definition.
I can't tell whether this defines the answer.
'piece of straw is perhaps thus' is the wordplay.
'piece of' indicates a hidden word.
'WISP' can be found hidden inside 'straw is perhaps thus'.
(Other definitions for wisp that I've seen before include "Small thin line of cloud or smoke" , "Slight trace" , "Small thin bunch/strand (of something)" , "Shred" , "Light streak of smoke or stray hair" .)