Wonderful fruit at the bottom of U-boat (7)
I believe the answer is:
sublime
'wonderful' is the definition.
(I've seen this before)
'fruit at the bottom of u-boat' is the wordplay.
'fruit' becomes 'lime' (lime is a kind of fruit).
'at the bottom of' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'u-boat' becomes 'sub' (German submarine).
'lime' after 'sub' is 'SUBLIME'.
(Other definitions for sublime that I've seen before include "Transcendent" , "Glorious; supreme" , "Of the most exalted or noble kind" , "Wonderful" , "Unparalleled, extreme" .)