Difficult storing energy that's picked up (5)
I believe the answer is:
heard
'picked up' is the definition.
(hearing is a kind of picking up)
'difficult storing energy' is the wordplay.
'difficult' becomes 'hard' ('hard' can be a synonym of 'difficult').
'storing' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'hard' placed around 'e' is 'HEARD'.
'that's' is the link.
(Other definitions for heard that I've seen before include "Was told of" , "Tried" , "Gave ear to" , "Attended to" , "Received aurally" .)