Family rely on vital supply (5,4)
I believe the answer is:
blood bank
'vital supply' is the definition.
(I've seen this before)
'family rely on' is the wordplay.
'family' becomes 'blood' (I've seen this in other clues).
'rely' becomes 'bank'.
'on' means one lot of letters go next to another.
'blood'+'bank'='BLOOD BANK'
(Other definitions for blood bank that I've seen before include "Store of vital fluid" , "vital supply" , "Reservoir for transfusions" .)