A measure of distress is terribly relative (6)
I believe the answer is:
sister
'relative' is the definition.
(sister is a kind of relative)
'a measure of distress is terribly' is the wordplay.
'a measure of' says the answer is hidden in the clue (a portion or measure of the clue).
'SISTER' can be found hidden inside 'distress is terribly'.
(Other definitions for sister that I've seen before include "woman in hospital" , "The nurse in charge" , "Senior nurse; relative" , "Tender" , "habits' - one who has them" .)