Giant swine held bottom up (7)
I believe the answer is:
goliath
'giant' is the definition.
(I know that giant can be written as goliath)
'swine held bottom up' is the wordplay.
'swine' becomes 'hog' (hog is a kind of swine).
'held' indicates putting letters inside.
'bottom' becomes 'tail' (synonyms).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'hog' enclosing 'tail' is 'htailog'.
'htailog' written backwards gives 'GOLIATH'.
(Other definitions for goliath that I've seen before include "Giant Philistine laid low by David" , "Biblical giant" , "Giant Philistine flattened by David" , "an unusually tall person" , "Giant Philistine killed by David" .)