A fat file I arranged to band together (9)
I believe the answer is:
affiliate
'band together' is the definition.
(thesaurus)
'a fat file i arranged' is the wordplay.
'arranged' indicates an anagram (letters rearranged).
'fat'+'file'+'i'='fatfilei'
'fatfilei' with letters rearranged gives 'ffiliate'.
'a'+'ffiliate'='AFFILIATE'
'to' acts as a link.
(Other definitions for affiliate that I've seen before include "Associate with a parent body" , "Become closely connected" , "Attach (to an organisation)" .)