Obtaining body fluids is extremely hard work (5,5,3,5)
I believe the answer is:
blood sweat and tears
'obtaining body' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'fluids is extremely hard work' is the wordplay.
I cannot quite understand how this works, but
'fluids' could be 'ars' (Ar is an example) and 'ars' is found within the answer.
'work' could be 'sweat' (sweat is a kind of work) and 'sweat' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for blood sweat and tears that I've seen is " Extremely hard work - unstinting effort".)