Female spy's forgetting thanks, greeting guru (9)
I believe the answer is:
maharishi
'guru' is the definition.
(I've seen this in another clue)
'female spy's forgetting thanks greeting' is the wordplay.
'female spy' becomes 'mata hari' (I've seen this before).
'forgetting' suggests deleting specific letters.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'greeting' becomes 'shi' (I can't justify this - if you can you should believe this answer much more).
'matahari' with 'ta' removed is 'mahari'.
'mahari'+'shi'='MAHARISHI'
Can you help me to learn more?
(Other definitions for maharishi that I've seen before include "leading teacher" , "Spiritual teacher" , "Hindu instructor" , "Religious leader" , "Seasoned beef-pork sausage" .)