Footwear gets small encouragement (5)
I believe the answer is:
boost
'encouragement' is the definition.
(synonyms)
'footwear gets small' is the wordplay.
'footwear' becomes 'boot' (boot is a kind of footwear).
'gets' indicates putting letters inside (gets can mean captures or absorbs).
'small' becomes 's' (abbreviation - e.g. clothes size).
'boot' enclosing 's' is 'BOOST'.
(Other definitions for boost that I've seen before include "Increase, amplify" , "Help to increase" , "promotion" , "Encourage or increase" , "Raise" .)