Secondary proposition from man docked on lunar module (5)
I believe the answer is:
lemma
'secondary proposition' is the definition.
(secondary proposition in an argument)
'man docked on lunar module' is the wordplay.
'docked' means to remove the last letter (to dock can mean to remove the tail of an animal).
'on' says to put letters next to each other.
'lunar module' becomes 'LEM' (Lunar Excursion Module from Apollo missions).
'man' with its last letter taken away is 'ma'.
'ma' after 'lem' is 'LEMMA'.
'from' acts as a link.
(Other definitions for lemma that I've seen before include "heading" , "Proposition" , "Subsidiary theorem" , "Subject's division" , "Intermediate proof" .)