The most common and most likely answer to this crossword clue is the 2 plus 3 letter word OF USE
I've seen this crossword clue in Financial Times
O
F
U
S
E
More clues leading to the result OF USE
- Nothing put together can be practical
- Old electrical item handy
- Helping of duck put together
- Handy round protective device
- Round electrical component is functional
- Practicable round safety device
- Handy old electrical component
- Handy, old circuit breaker
- Ordinary circuit breaker may be helpful
- Round safety device that’s practicable
Buddy explains!
The straight part
We know that "functional" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case functional = of use.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.