Boxers perhaps boxing 50 spies (6)
I believe the answer is:
plants
'spies' is the definition.
(plant can mean someone planted in order to spy)
'boxers perhaps boxing 50' is the wordplay.
'boxers perhaps' becomes 'pants' (boxers are a type of underwear).
'boxing' means one lot of letters goes inside another ('box' can mean contain, as in to put in a box).
'50' becomes 'l' (Roman numeral).
'pants' placed around 'l' is 'PLANTS'.
(Other definitions for plants that I've seen before include "climbers?" , "Puts into the ground to grow" , "Settles" , "Living organisms without power of movement" , "Sets seeds in the ground" .)