Gruff son watched what his deferential sister might have done? (9)
I believe the answer is:
curtseyed
'what his deferential sister might have done?' is the definition.
(a woman might curtsey deferentially)
'gruff son watched' is the wordplay.
'gruff' becomes 'curt' (both can mean brusque or rude).
'son' becomes 's' (genealogical abbreviation for son).
'watched' becomes 'eyed' (I've seen this before).
'curt'+'s'+'eyed'='CURTSEYED'
(Other definitions for curtseyed that I've seen before include "Bobbed - bent the knee" , "what respectful girl did?" , "Showed deference" .)