Timid female wearing a sack (6)
I believe the answer is:
afraid
'timid' is the definition.
('afraid' is associated in meaning with 'timid')
'female wearing a sack' is the wordplay.
'female' becomes 'f' (abbreviation).
'wearing' is an insertion indicator.
'sack' becomes 'raid' (I've seen this in other clues).
'a'+'raid'='araid'
'f' going inside 'araid' is 'AFRAID'.
(Other definitions for afraid that I've seen before include "Frightened" , "Fearful, anxious" , "Regretfully" , "Scared - politely regretful" , "Filled with apprehension" .)