Expedition retains openings for trainee explorers (5)
I believe the answer is:
haste
'expedition' is the definition.
(expedition is expediting, doing something quickly)
'retains openings for trainee explorers' is the wordplay.
'retains' becomes 'has' (have can mean to retain or keep).
'openings for' suggests taking the first letters (the letters which open the words).
The initial letters of 'trainee explorers' is 'te'.
'has'+'te'='HASTE'
(Other definitions for haste that I've seen before include "The more of it, the less speed, it is said" , "Hurriedness" , "Promptitude" , "More of this proverbially slows you down" , "Leader abandons modest" .)