Fuss when sibling loses a right (6)
I believe the answer is:
bother
'fuss' is the definition.
(I know that fuss can be written as bother)
'sibling loses a right' is the wordplay.
'sibling' becomes 'brother' (male sibling).
'loses' suggests deleting specific letters.
'a right' becomes 'r' (common abbreviation).
'brother' with 'r' taken out is 'BOTHER'.
'when' acts as a link.
(Other definitions for bother that I've seen before include "Cause worry or annoyance" , "Worry; fuss" , "Pester, aggravate" , "Annoy or pester" , "Discommode, annoy" .)