Cosmetic treatment - a lot of detail, I allow, having removed base (6)
I believe the answer is:
facial
'cosmetic treatment' is the definition.
(I've seen this before)
'a lot of detail i allow having removed base' is the wordplay.
'a lot of detail i' becomes 'facia' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'allow' becomes 'le' (I've seen this in other clues).
'having removed' suggests deleting specific letters.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'le' with 'e' taken away is 'l'.
'facia'+'l'='FACIAL'
Can you help me to learn more?
(Other definitions for facial that I've seen before include "Beauty treatment, maybe mudpack" , "Cosmetic treatment" , "Beauty treatment for the visage" , "Type of beauty treatment" , "Beauty treatment for front of head" .)