Relative takes a bit of tweed for top of pillbox hat (7)
I believe the answer is:
stetson
'top of pillbox hat' is the definition.
'stetson' can be an answer for 'hat' (Stetson is an example). I'm not sure about the 'top of pillbox' bit.
'relative takes a bit of tweed' is the wordplay.
'relative' becomes 'setson' (I can't explain this - if you can you should believe this answer much more).
'takes' is an insertion indicator.
'a bit of' indicates taking the first letters.
The first letter of 'tweed' is 't'.
'setson' placed around 't' is 'STETSON'.
'for' acts as a link.
(Other definitions for stetson that I've seen before include "St Neots (anag.)" , "Hat for country-and-western singers, Texan oil magnates, etc." , "A wide-brimmed cowboy hat" , "Hat with broad brim and high crown" , "Man's hat with wide brim and high crown" .)