3 down
Belly
squeezed
by
fifth
columnist
in
squat
(6)
Belly squeezed by fifth columnist in squat (6)
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The definition is at the end of the clue.
How the clue works
"Squat" is the definition here, meaning short and stocky, same as STUMPY. For the wordplay, take TUM for "belly" and squeeze it inside SPY (that's your "fifth columnist", someone secretly aiding the enemy), giving you S-TUM-PY.