Family bathroom designed primarily to accommodate toilet (5)
I believe the answer is:
blood
'family' is the definition.
(I've seen this in other clues)
'bathroom designed primarily to accommodate toilet' is the wordplay.
'primarily' suggests taking the first letters.
'to accommodate' is an insertion indicator.
'toilet' becomes 'loo' (loo can informally mean a toilet).
The initial letters of 'bathroom designed' is 'bd'.
'bd' placed around 'loo' is 'BLOOD'.
(Other definitions for blood that I've seen before include "Kinship; temperament" , "It contains plasma and platelets" , "Red fluid" , "Become the duty of" , "It's said to be thicker than water" .)