Soldiers advanced without being persistent (10)
I believe the answer is:
relentless
'persistent' is the definition.
('relentless' can be a synonym of 'persistent')
'soldiers advanced without' is the wordplay.
'soldiers' becomes 're' (Royal Engineers**).
'advanced' becomes 'lent' (money).
'without' becomes 'less'.
're'+'lent'+'less'='RELENTLESS'
'being' acts as a link.
(Other definitions for relentless that I've seen before include "Allowing no respite" , "Uncompromising" , "Oppressively constant; inflexible" , "Persistent" , "Dogged" .)