Gosh! I'm not sure what steals Jonathan's heart? (5)
I believe the answer is:
corer
'what steals jonathan's heart?' is the definition.
(Jonathan is a type of apple)
'gosh i'm not sure' is the wordplay.
'gosh' becomes 'cor' (I've seen this in other clues).
'i'm not sure' becomes 'er' (expression of hesitation).
'cor'+'er'='CORER'
(Other definitions for corer that I've seen before include "(Eg, apple) cutter" , "kitchen implement?" , "Device used on apples" .)