Pattern of suit jacket from Oxfam that's sent back (5)
I believe the answer is:
motif
'pattern' is the definition.
(I know that motif is a type of pattern)
'suit jacket from oxfam that's sent back' is the wordplay.
'suit' becomes 'fit' (synonyms).
'jacket from' means to remove the middle letters (the letters which form the word\'s case or jacket).
'that's sent back' is a reversal indicator.
'oxfam' with its middle removed is 'om'.
'fit'+'om'='fitom'
'fitom' back-to-front is 'MOTIF'.
'of' is the link.
(Other definitions for motif that I've seen before include "Distinctive theme in art" , "Recurrent theme as in musical work" , "Decorative design" , "Theme in music or book" , "Recurring image in design" .)