Certainly not loose leggings (6)
I believe the answer is:
tights
'certainly not loose leggings' is the definition.
'tights' can be an answer for 'leggings' (I have seen 'Female leggings' mean 'tights' so perhaps 'leggings' could also mean 'tights'). I'm not certain of the 'certainly not loose' bit.
'not loose leggings' is the wordplay.
'not loose' becomes 'tight' ('tight' is the opposite to 'loose').
'leggings' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'tight'+'s'='TIGHTS'
Can you help me to learn more?
(Other definitions for tights that I've seen before include "Hose worn by dancers" , "Leg-wear for women" , "layer of clothing" , "Hose coverings from waist to feet" , "Female leggings" .)