Keep shortened shape with hair (8)
I believe the answer is:
fortress
'keep' is the definition.
(I've seen this in another clue)
'shortened shape with hair' is the wordplay.
'shortened' means to remove the last letter.
'shape' becomes 'form' (synonyms).
'with' says to put letters next to each other.
'hair' becomes 'tress' (tress is a kind of hair).
'form' with its last letter removed is 'for'.
'for'+'tress'='FORTRESS'
(Other definitions for fortress that I've seen before include "Military stronghold" , "it's hard to break into" , "Protective structure" , "Strong point" , "Fortified defensive structure" .)