There's many a pointer as to how to grow one (6)
I believe the answer is:
marrow
'to how to grow one' is the definition.
I can't judge whether this defines the answer.
'there's many a pointer' is the wordplay.
'there's' means one lot of letters go next to another.
'many' becomes 'm' (Roman numeral for a thousand).
'a pointer' becomes 'arrow' ('arrow' can be a synonym of 'pointer').
'm'+'arrow' is 'MARROW'.
'as' is the link.
Can you help me to learn more?
(Other definitions for marrow that I've seen before include "Large green vegetable" , "It fills bone cavities" , "Bone filler" , "Large courgette" , "Connective tissue in bones" .)