The most common and most likely answer to this crossword clue is the 3 letter word SAP
S
A
P
More clues leading to the result SAP
- Dad's served up juice
- Reprimand avoided by learner, a poor student
- Weaken juice
- Step up energy to undermine energy-efficiency indicator
- Exhaust — it’s quiet
- Gullible person in France not returning
- Dad’s knocked over fluid
- Energy old man's needed after retiring
- Juice quietly taken down in Bath, for example?
- Juice the old man's knocked over
Buddy explains!

The straight part
We know that "energy/to undermine/energy-efficiency indicator" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case energy/to undermine/energy-efficiency indicator = sap.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SAP mean?
SAP noun- The juice of plants of any kind, especially the ascending and descending juices or circulating fluid essential to nutrition.
- The sapwood, or alburnum, of a tree.
- Any juice.
- Vitality.
- A naive person; a simpleton
- To drain, suck or absorb from (tree, etc.).
- To exhaust the vitality of.
- A short wooden club; a leather-covered hand weapon; a blackjack.
- To strike with a sap (with a blackjack).
- A narrow ditch or trench made from the foremost parallel toward the glacis or covert way of a besieged place by digging under cover of gabions, etc.
- To subvert by digging or wearing away; to mine; to undermine; to destroy the foundation of.
- To pierce with saps.
- To make unstable or infirm; to unsettle; to weaken.
- To gradually weaken.
- To proceed by mining, or by secretly undermining; to execute saps.