Gentle criminal a guard left (7)
I believe the answer is:
gradual
'gentle' is the definition.
(similar in meaning)
'criminal a guard left' is the wordplay.
'criminal' is an anagram indicator.
'left' becomes 'al' (I've seen this in other clues).
'guard' is an anagram of 'gradu'.
'gradu'+'al'='GRADUAL'
(Other definitions for gradual that I've seen before include "Not steep or abrupt" , "Progressive" , "Bit by bit" , "Progressing by degrees" , "Developing in stages" .)