Teach outside, below boom (7)
I believe the answer is:
thunder
'boom' is the definition.
(I know that boom can be written as thunder)
'teach outside below' is the wordplay.
'outside' means to remove the middle letters.
'below' becomes 'under' (synonyms).
'teach' with its middle removed is 'th'.
'th'+'under'='THUNDER'
(Other definitions for thunder that I've seen before include "Storm noise" , "Atmospheric booms" , "Stormy peal" , "Weather featuring with 1" , "Loud and prolonged noise in the sky" .)