The most common and most likely answer to this crossword clue is the 6 letter word SMELLS
I've seen this crossword clue in The Guardian
S
M
E
L
L
S
More clues leading to the result SMELLS
Buddy explains!

The straight part
We know that "seems suspicious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seems suspicious = smells.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 SMELLS mean?
SMELLS noun- A sensation, pleasant or unpleasant, detected by inhaling air (or, the case of water-breathing animals, water) carrying airborne molecules of a substance.
- The sense that detects odours.
- A conclusion or intuition that a situation is wrong, more complex than it seems, or otherwise inappropriate.
- To sense a smell or smells.
- Followed by like or of if descriptive: to have a particular smell, whether good or bad.
- (without a modifier) To smell bad; to stink.
- To have a particular tincture or smack of any quality; to savour.
- To exercise sagacity.
- To detect or perceive; often with out.
- To give heed to.