Brief rest in rocky site in outskirts of sierra (6)
I believe the answer is:
siesta
'brief rest' is the definition.
(I've seen this in another clue)
'rocky site in outskirts of sierra' is the wordplay.
'rocky' is an anagram indicator.
'in' is an insertion indicator.
'outskirts of' means to remove the middle letters.
'sierra' with its centre removed is 'sa'.
'site' is an anagram of 'iest'.
'iest' placed inside 'sa' is 'SIESTA'.
'in' is the link.
(Other definitions for siesta that I've seen before include "Forty winks" , "Afternoon nap in Cadiz" , "Afternoon nap in a warm climate" , "After lunch nap" , "Rest of Spain" .)