Developing hot legs and tush, silly (11)
I believe the answer is:
thoughtless
'silly' is the definition.
(thesaurus)
'developing hot legs and tush' is the wordplay.
'developing' is an anagram indicator.
'and' says to put letters next to each other (I've seen this in other clues).
'hot'+'legs'='hotlegs'
'hotlegs' put after 'tush' is 'tushhotlegs'.
'tushhotlegs' is an anagram of 'THOUGHTLESS'.
(Other definitions for thoughtless that I've seen before include "Lacking consideration" , "Inconsiderate - reckless" , "Showing little consideration for others" , "Insensitive" , "Uncaring" .)