Advertise footwear protecting back of legs (5)
I believe the answer is:
boost
'advertise' is the definition.
(thesaurus)
'footwear protecting back of legs' is the wordplay.
'footwear' becomes 'boot' (boot is a kind of footwear**).
'protecting' means one lot of letters goes inside another.
'back of' says to take the final letters.
The final letter of 'legs' is 's'.
'boot' going around 's' is 'BOOST'.
(Other definitions for boost that I've seen before include "lift" , "Raise" , "Promote" , "Encourage or increase" , "Encourage; push up" .)