Tense, white... about to burst out (5)
I believe the answer is:
erupt
'burst out' is the definition.
(synonyms)
'tense white about' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'white' becomes 'pure' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is a reversal indicator.
't'+'pure'='tpure'
'tpure' written backwards gives 'ERUPT'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for erupt that I've seen before include "Eject lava" , "Break out dramatically" , "Break out suddenly and violently" , "Emit lava" , "Burst forth, like 25" .)