The head of the family isn't weak (5)
I believe the answer is:
faint
'weak' is the definition.
(I've seen this before)
'the head of the family isn't' is the wordplay.
'the head of' indicates taking the first letters.
'isn't' becomes 'aint' (I've seen this before).
The first 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" .)