An advocate by day, that woman in a tent, scratching head (8)
I believe the answer is:
adherent
'an advocate' is the definition.
(I've seen this before)
'day that woman in a tent scratching head' is the wordplay.
'day' becomes 'd'.
'that woman' becomes 'her' (I've seen this before**).
'in' is an insertion indicator.
'scratching head' means to remove the first letter.
'd'+'her'='dher'
'tent' with its initial letter removed is 'ent'.
'a'+'ent'='aent'
'dher' going into 'aent' is 'ADHERENT'.
'by' is the link.
(Other definitions for adherent that I've seen before include "lover" , "Disciple, follower" , "One supporting" , "Freak" , "Devotee" .)