The head of the family isn't weak (5)
I believe the answer is:
faint
'weak' is the definition.
(similar in meaning)
'the head of the family isn't' is the wordplay.
'the head of' suggests taking the first letters.
'isn't' becomes 'aint' (non-standard version of 'isn't').
The initial letter of 'family' is 'f'.
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "woozy" , "Swoon - pale" , "hard to hear" , "dull" , "Lacking clarity" .)