It's pointless to tip half the peas into a new tin (5)
I believe the answer is:
inept
'it's pointless' is the definition.
(I've seen this before)
'tip half the peas into a new tin' is the wordplay.
'tip' says the letters should be written backwards (I've seen 'tipped' mean this).
'half the' means to take half the letters.
'into' indicates putting letters inside.
'a new' is an anagram indicator.
'peas' with half the letters taken is 'pe'.
'pe' in reverse letter order is 'ep'.
'tin' anagrammed gives 'int'.
'ep' going into 'int' is 'INEPT'.
'to' is the link.
(Other definitions for inept that I've seen before include "Amateurish" , "Wanting" , "Unsuitable, clumsy" , "Silly or senseless" , "Ham-fisted" .)