Sailor, ahead of a giddy aunt, carrying large spider (9)
I believe the answer is:
'spider' is the definition.
(tarantula is a kind of spider)
'sailor ahead of a giddy aunt carrying large' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor**).
'ahead of' means one lot of letters go next to another (I've seen this in other clues).
'giddy' indicates anagramming the letters.
'carrying' is an insertion indicator.
'large' becomes 'l'.
'aunt' is an anagram of 'ntua'.
'ntua' enclosing 'l' is 'ntula'.
(Other definitions for tarantula that I've seen before include "A large hairy tropical spider" , "spider to appear" , "Breed of spider" , "Creepy-crawly" , "Leggy creature" .)