Particle new to Planck, and not recalled after brief time (7)
I believe the answer is:
neutron
'particle' is the definition.
(neutron is a kind of particle)
'new to planck and not recalled after brief time' is the wordplay.
I cannot really see how this works, but
'new' could be 'n' (common abbreviation eg NT for New Testament) and 'n' is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
'time' could be 't' (abbreviation) and 't' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for neutron that I've seen before include "Elementary particle with zero charge" , "Particle that decays when released" , "Charged particle" , "Chargeless particle" , "one isn't charged" .)