The most common and most likely answer to this crossword clue is the 7 letter word TINNIER
I've seen this crossword clue in The Times
T
I
N
N
I
E
R
More clues leading to the result TINNIER
Buddy explains!

The straight part
We know that "Comparatively thin, metallic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case comparatively thin, metallic = tinnier.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 TINNIER mean?
TINNIER adjective- Of or pertaining to or resembling tin.
- Pertaining to the thinness and cheapness of tin or similar-looking metals, in contrast to a heavier, more valuable metal.
- Pertaining to a thin, unpleasant sound recalling that of tin being rapped.