Inspire soldiers to swim outside (7)
I believe the answer is:
breathe
'inspire' is the definition.
(I know that inspire has the is a more specific form of the action breathe)
'soldiers to swim outside' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'to swim' becomes 'bathe' (bathe can mean to swim).
'outside' means one lot of letters goes inside another.
're' going into 'bathe' is 'BREATHE'.
(Other definitions for breathe that I've seen before include "Take into mouth" , "use aqualungs, perhaps" , "as wine may be allowed to do" , "Absorb air" , "Inhale and exhale" .)