Shape of longbow almost entirely bent (6)
I believe the answer is:
oblong
'shape' is the definition.
(oblong is a kind of shape)
'longbow almost entirely bent' is the wordplay.
'almost entirely' means to remove the last letter.
'bent' is an anagram indicator.
'longbow' with its last letter taken off is 'longbo'.
'longbo' anagrammed gives 'OBLONG'.
'of' is the link.
(Other definitions for oblong that I've seen before include "Plane figure elongated in one direction" , "Rectangular but not square" , "book form?" , "Oval" , "Something with an elongated rectangular shape" .)