I can net puzzle – it’s old! (7)
I believe the answer is:
ancient
'old' is the definition.
(similar in meaning)
'i can net puzzle' is the wordplay.
'puzzle' is an anagram indicator.
'i'+'can'+'net'='icannet'
'icannet' anagrammed gives 'ANCIENT'.
'it's' is the link.
(Other definitions for ancient that I've seen before include "over the hill" , "Very old or belonging to times past" , "Archaic" , "very old boy?" , "Extremely old" .)