They protect young things and elderly bodies beyond 100 (4,6)

'they protect young things' is the definition.
(type of garden device which protects young plants)

'elderly bodies beyond 100' is the wordplay.
'elderly' becomes 'old' ('old' can be similar in meaning to 'elderly').
'bodies' becomes 'frames' (frame is a kind of body**).
'beyond' means one lot of letters go next to another.
'f00' becomes 'c' ('C' can be a synonym of 'f00').
'old'+'frames'='oldframes'
'oldframes' after 'c' is 'COLD FRAMES'.

'and' acts as a link.