Some quantity of light pressure blistering leg (6)
I believe the answer is:
photon
'some quantity of light' is the definition.
(light particle)
'pressure blistering leg' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'blistering' becomes 'hot' (similar in meaning).
'leg' becomes 'on' (cricket term, fielding positions).
'p'+'hot'+'on'='PHOTON'
(Other definitions for photon that I've seen before include "Quantum of electromagnetic radiation" , "Quantum particle of light" , "A little light" , "Tiny bit of light" .)