Landing (4)
'landing' is the definition.
(pier is a kind of landing)
This is all the clue.
(Other definitions for pier that I've seen before include "Support for two adjacent bridge spans" , "Structure extending seawards" , "Mole, wall between windows" , "Building projecting into the sea" , "such as you might see at the seaside" .)
'landing' is the definition.
(touching down is a kind of landing)
This is the entire clue.