Call for nurse before onset of tummy trouble (6)
I believe the answer is:
entail
'call for' is the definition.
(I've seen this before)
'nurse before onset of tummy trouble' is the wordplay.
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'before' means one lot of letters go next to another.
'onset of' suggests taking the first letters.
'trouble' becomes 'ail' (synonyms).
The initial letter of 'tummy' is 't'.
'en'+'t'+'ail'='ENTAIL'
(Other definitions for entail that I've seen before include "Invlove or necessitate" , "Cause" , "Inevitably result in" , "Bring on, involve" , "Inalienable estate" .)