A spirit? The old man imbibes rye, mostly (5)
I believe the answer is:
dryad
'a spirit?' is the definition.
(type of spirit in Greek myth)
'old man imbibes rye mostly' is the wordplay.
'old man' becomes 'dad' (both can mean one's father).
'imbibes' is an insertion indicator.
'mostly' means to remove the last letter.
'rye' with its final letter removed is 'ry'.
'dad' enclosing 'ry' is 'DRYAD'.
'the' acts as a link.
(Other definitions for dryad that I've seen before include "one stuck in a tree" , "Tree nymph" , "Beautiful young woman" , "Woodland nymph" , "Woodland deity" .)