The most common and most likely answer to this crossword clue is the 7 letter word PINHEAD
P
I
N
H
E
A
D
More clues leading to the result PINHEAD
Buddy explains!

The straight part
We know that "dumbbell" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dumbbell = pinhead.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 PINHEAD mean?
PINHEAD noun- The head of a pin. (Frequently used in size comparisons.)
- An ignorant, naive, foolish, or stupid person.
- A telemark skier.
- A human head that is unusually tapered or small, often due to microcephaly, or a person with that trait. Often promoted in freak shows as "human pinheads".
- (pet stores) A newborn cricket used as food for pets.
- (mycoculture) The immature juvenile fruiting body of a mushroom prior to its gills opening.