Plain girl's first in cool sex tips (9)
I believe the answer is:
serengeti
'plain' is the definition.
(I know that Serengeti is a plain)
'girl's first in cool sex tips' is the wordplay.
'first' says to take the initial letters.
'in' means one lot of letters goes inside another.
'cool' becomes 'serene' (I've seen this in another clue).
'sex' becomes 'it' (a common euphemism).
'tips' says the letters should be written backwards (I've seen 'tipped' mean this).
The initial letter of 'girl' is 'g'.
'it' back-to-front is 'ti'.
'g' going into 'serene' is 'serenge'.
'serenge'+'ti'='SERENGETI'
(Other definitions for serengeti that I've seen before include "Tanzanian plain renowned for its wildlife" , "Vast plain" , "African plain noted for wildlife" , "where lion holds sway?" , "reserve for game" .)