Proposition boy soldiers misunderstood at first (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'boy soldiers misunderstood at first' is the wordplay.
'boy' becomes 'theo' (**).
'soldiers' becomes 're' (Royal Engineers).
'at first' suggests taking the first letters.
The first letter of 'misunderstood' is 'm'.
'theo'+'re'+'m'='THEOREM'
(Other definitions for theorem that I've seen before include "Proven hypothesis" , "Logical proposition" , "Algebraic rule" , "this has been established" , "Deduced formula" .)