Grannie so loose, working out (9)
I believe the answer is:
reasoning
'working out' is the definition.
'grannie so loose' is the wordplay.
'loose' indicates an anagram (letters are loosened from their original positions).
'grannie'+'so'='grannieso'
'grannieso' is an anagram of 'REASONING'.
(Other definitions for reasoning that I've seen before include "Thinking that is coherent and logical" , "Debating" , "Train of thought" , "Logical thought process" , "argument" .)