Maxim gun soldiers overturned encased in lead (7)
I believe the answer is:
proverb
'maxim' is the definition.
(I've seen this before)
'gun soldiers overturned encased in lead' is the wordplay.
'gun' becomes 'rev' (as in gunning or revving up a car engine).
'soldiers' becomes 'OR' (military abbreviation for other ranks).
'overturned' shows that the letters should be reversed in order.
'encased in' is an insertion indicator.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'rev'+'or'='revor'
'revor' written backwards gives 'rover'.
'rover' inserted into 'pb' is 'PROVERB'.
(Other definitions for proverb that I've seen before include "encapsulate wisdom" , "Well-known statement incorporating general truth" , "Aphorism" , "Short saying embodying a supposed truth" , "Old saying, perhaps by Solomon?" .)