Weak start to fashionable bar (5)
I believe the answer is:
frail
'weak' is the definition.
('frail' can be similar in meaning to 'weak')
'start to fashionable bar' is the wordplay.
'start to' indicates taking the first letters.
'bar' becomes 'rail' (rail is a kind of bar).
The first letter of 'fashionable' is 'f'.
'f'+'rail'='FRAIL'
(Other definitions for frail that I've seen before include "Easily destroyed" , "Slight" , "Old woman" , "Struggling to get about" , "Weak and infirm" .)