Burst of letters from higher up, thematically (5)
I believe the answer is:
erupt
'burst' is the definition.
(erupting is a kind of bursting)
'letters from higher up thematically' is the wordplay.
'letters from' says the answer is hidden in the clue (a selection of letters from the clue).
'ERUPT' is hidden in the letters of 'higher up thematically'.
'of' is the link.
(Other definitions for erupt that I've seen before include "What canines do" , "go up" , "be active" , "Break through (as a volcano)" , "Discharge" .)