Set down soldiers, infantry principally, in training camp (7)
I believe the answer is:
deposit
'set down' is the definition.
(I've seen this before)
'soldiers infantry principally in training camp' is the wordplay.
'principally' suggests taking the first letters.
'in' is an insertion indicator.
'training camp' becomes 'depot' (I can't explain this - if you can you should believe this answer much more).
The first letters of 'soldiers infantry' is 'si'.
'si' inserted within 'depot' is 'DEPOSIT'.
Can you help me to learn more?
(Other definitions for deposit that I've seen before include "Lay down" , "fur, possibly" , "Money given as security" , "part-payment" , "Place" .)