Above it all, in a way (5)
'above it all in a way' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tallest that I've seen before include "most implausible" , "Of greatest height" , "Sell tat (anag.)" , "Of greatest vertical extent" , "Of extreme height" .)
(Other definitions for aloft that I've seen before include "Float (anag.)" , "up the mast" , "at high altitude" , "Above - on top" , "To great height" .)