Mendicant to carry about a couple of gallons (6)
I believe the answer is:
beggar
'mendicant' is the definition.
('beggar' can be a synonym of 'mendicant')
'carry about a couple of gallons' is the wordplay.
'carry' becomes 'bear' (bearing is a kind of carrying).
'about' means one lot of letters goes inside another.
'a couple of' shows that letters should be duplicated.
'gallons' becomes 'g'.
'g' duplicated is 'gg'.
'bear' enclosing 'gg' is 'BEGGAR'.
'to' acts as a link.
(Other definitions for beggar that I've seen before include "Solicitor" , "One cannot be a chooser" , "He can't be a chooser, proverbially" , "Tramp" , "Pauper who seeks alms" .)