Group keeping base spotless (5)
I believe the answer is:
clean
'spotless' is the definition.
(similar in meaning)
'group keeping base' is the wordplay.
'group' becomes 'clan'.
'keeping' means one lot of letters goes inside another (some letters kept inside).
'base' becomes 'e' (Euler's number - base of natural logarithms).
'clan' going around 'e' is 'CLEAN'.
(Other definitions for clean that I've seen before include "Without stain" , "Unsullied" , "Innocent" , "Non-polluting" , "Clear (of drugs)" .)