Blow up, being rather uptight? Not entirely (5)
I believe the answer is:
erupt
'blow up' is the definition.
(I've seen this before)
'being rather uptight? not entirely' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'ERUPT' is hidden in the letters of 'being rather uptight'.
(Other definitions for erupt that I've seen before include "Eject lava" , "Emit lava" , "Break through (as a volcano)" , "suddenly burst out" , "Explode" .)