The weak half of a bar (5)
I believe the answer is:
frail
'the weak' is the definition.
(thesaurus)
'half of a bar' is the wordplay.
'half' indicates taking half.
'a bar' becomes 'rail' (rail is a kind of bar**).
'of' with half the letters taken is 'f'.
'f'+'rail'='FRAIL'
(Other definitions for frail that I've seen before include "Puny" , "Delicate, fragile" , "Decrepit" , "Easily destroyed" , "Weak and infirm" .)