Dogged soldiers fast no more (10)

I believe the answer is:
relentless
'dogged' is the definition.
(I've seen this before)
'soldiers fast no more' is the wordplay.
'soldiers' becomes 're' (Royal Engineers**).
'fast' becomes 'lent' (I've seen this before).
'no more' becomes 'less' (I've seen this in other clues).
're'+'lent'+'less'='RELENTLESS'
(Other definitions for relentless that I've seen before include "Uncompromising" , "Unyielding - implacable" , "Allowing no respite" , "Harsh, inflexible" , "Inexorable" .)