The most common and most likely answer to this crossword clue is the 4 letter word VERY
I've seen this crossword clue in The Daily Telegraph
V
E
R
Y
More clues leading to the result VERY
Buddy explains!
The straight part
We know that "Extremely/inventive signalman" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely/inventive signalman = very.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 VERY mean?
VERY adjective- True, real, actual.
- The same; identical.
- With limiting effect: mere.
- To a great extent or degree.
- Conforming to fact, reality or rule; true.
- (with superlatives) Used to firmly establish that nothing else surpasses in some respect.