"I'll second that!" (5)
I believe the answer is:
ditto
'i'll second that' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ditto that I've seen before include "that which has been said" , "Repeat the word above" , "Likewise" , "Mark used to indicate the word above it should be repeated" , "Same again, indicated by two dots" .)