Nurse is flipping over poop, mostly (6)
I believe the answer is:
sister
'nurse' is the definition.
(a female nurse might be called sister)
'flipping over poop mostly' is the wordplay.
'flipping over' becomes 'si' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'poop' becomes 'stern' (synonyms).
'mostly' means to remove the last letter (most but not all of the word).
'stern' with its final letter removed is 'ster'.
'si'+'ster'='SISTER'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for sister that I've seen before include "Nun's title" , "trade union member?" , "woman in hospital" , "Relative in hospital?" , "A kinswoman" .)