Family on radio knowing about climbing sierra? (7)
I believe the answer is:
archers
'family on radio' is the definition.
(in BBC radio drama The Archers)
'knowing about climbing sierra?' is the wordplay.
'knowing' becomes 'arch' (I've seen this before).
'about' becomes 're' (regarding).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'sierra?' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
're' written backwards gives 'er'.
'arch'+'er'+'s'='ARCHERS'
(Other definitions for archers that I've seen before include "old soldiers?" , "Old combatants" , "Agincourt soldiers?" , "Their sport is the bow and arrow" , "Experts with bows and arrows" .)