Middle of masthead below boom (7)
I believe the answer is:
thunder
'boom' is the definition.
(I know that boom can be written as thunder)
'middle of masthead below' is the wordplay.
'middle of' means to look at the middle letters.
'below' becomes 'under' (synonyms).
The middle letters of 'masthead' are 'th'.
'th'+'under'='THUNDER'
(Other definitions for thunder that I've seen before include "Stormy peal" , "Rail" , "Meteorological rumblings" , "Loud rumbling noise" , "It accompanies lightning" .)