Gallon drunk by pal, powerless from the start (3,5)
I believe the answer is:
all along
'from the start' is the definition.
'gallon drunk by pal' is the wordplay.
'drunk' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'pal' becomes 'al' (I can't justify this - if you can you should give a lot more credence to this answer).
'gallon' with letters rearranged gives 'lalong'.
'lalong' put after 'al' is 'ALL ALONG'.
'powerless' is the link.
This may not be right. It may belong to another bit of the clue.
(Other definitions for all along that I've seen before include "Consistently over a period of time" , "Covering the entire distance" , "From the beginning" , "What across answers have always been" , "From the outset" .)