Uncompromising army corps not so fast invading? (10)
I believe the answer is:
relentless
'uncompromising' is the definition.
(I've seen this in another clue)
'army corps not so fast invading?' is the wordplay.
'army corps' becomes 'RE' (Royal Engineers).
'not so' becomes 'less' (eg 'not so big' means 'less big').
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'invading?' means one lot of letters goes inside another.
're'+'less'='reless'
'reless' enclosing 'lent' is 'RELENTLESS'.
(Other definitions for relentless that I've seen before include "Allowing no respite" , "Unyielding - implacable" , "Inexorable" , "Dogged" , "sustained" .)