Small amounts of energy could be measured numerically, they say, if I'd short ammeter (6)
I believe the answer is:
quanta
'small amounts of energy' is the definition.
(plural of quantum)
'i'd short ammeter' is the wordplay.
'i'd' becomes 'quant' (I can't justify this - if you can you should believe this answer much more).
'short' indicates taking the first letters.
The initial letter of 'ammeter' is 'a'.
'quant'+'a'='QUANTA'
'could be measured numerically they say if' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for quanta that I've seen before include "Smallest discrete amounts" , "Smallest amounts in physics" , "Packets of energy" .)