Really long to inhabit tent in retirement (5)
I believe the answer is:
truly
'really' is the definition.
(synonyms)
'long to inhabit tent in retirement' is the wordplay.
'long' becomes 'l'.
'to inhabit' means one lot of letters goes inside another.
'tent' becomes 'yurt' (I've seen this before).
'in retirement' says the letters should be written in reverse (retire can mean to go back or backwards).
'l' going within 'yurt' is 'ylurt'.
'ylurt' reversed gives 'TRULY'.
(Other definitions for truly that I've seen before include "Without equivocation (Yours ___ )" , "Indeed" , "with accuracy" , "In reality" , "Verily" .)