Old hand in part shepherds one learner: it may bear fruit (3,4)
I believe the answer is:
oil palm
'it may bear fruit' is the definition.
(fruit tree)
'old hand in part shepherds one learner' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'hand in part' becomes 'palm' (part of a hand).
'shepherds' means one lot of letters goes inside another (inserted letters are shepherded or guarded).
'one' becomes 'i' (Roman numeral).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'o'+'palm'='opalm'
'i'+'l'='il'
'opalm' placed around 'il' is 'OIL-PALM'.
(Other definitions for oil palm that I've seen before include "Give bribe, perhaps" , "source of cosmetic ingredient" , "Fat-yielding tree" .)