The most common and most likely answer to this crossword clue is the 7 letter word TRIODES
T
R
I
O
D
E
S
Buddy explains!

The straight part
We know that "Valves" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case valves = triodes.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 TRIODES mean?
TRIODES noun- A thermionic valve containing an anode, a cathode, and a control grid; small changes to the charge on the grid control the flow from cathode to anode, which makes amplification possible.