Prepare unit to invade landing stage (7)
I believe the answer is:
pioneer
'prepare' is the definition.
(I know that prepare is a more specific form of pioneer)
'unit to invade landing stage' is the wordplay.
'unit' becomes 'one' (synonyms).
'to invade' is an insertion indicator (inserted letters invade the word).
'landing stage' becomes 'pier' (structure by the sea).
'one' going within 'pier' is 'PIONEER'.
(Other definitions for pioneer that I've seen before include "One entering new territory" , "make the first move!" , "Innovate" , "Innovator; first settler" , "Innovator; early settler" .)