Rest one's back, having a specially designed seat (6)
I believe the answer is:
siesta
'rest' is the definition.
(I've seen this before)
'one's back having a specially designed seat' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'back' shows that the letters should be reversed in order.
'having' is a charade indicator (letters next to each other).
'a specially designed seat' becomes 'esta' (I am not sure about this - if you are sure you should believe this answer much more).
'is' reversed gives 'si'.
'si'+'esta'='SIESTA'
Can you help me to learn more?
(Other definitions for siesta that I've seen before include "Afternoon snooze" , "Daytime nap" , "Afternoon sleep" , "Early afternoon nap" , "Afternoon nap in a warm climate" .)