Test old tripe about capturing soldiers (10)
I believe the answer is:
experiment
'test' is the definition.
(I've seen this before)
'old tripe about capturing soldiers' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'about' indicates an anagram.
'capturing' means one lot of letters goes inside another.
'soldiers' becomes 'men' (men can mean a group of soldiers).
'tripe' is an anagram of 'perit'.
'perit' placed around 'men' is 'periment'.
'ex'+'periment'='EXPERIMENT'
(Other definitions for experiment that I've seen before include "scientific investigation" , "Trial" , "Controlled test" .)