Random tests in place leading to arrests (4-6)

'random tests' is the definition.
(a spot-check is a random or unannounced inspection)

'place leading to arrests' is the wordplay.
'place' becomes 'spot' (synonyms).
'leading to' is a charade indicator (letters next to each other) (some letters go before or lead others).
'arrests' becomes 'checks' ('check' can be a synonym of 'arrest').
'spot'+'checks'='SPOT-CHECKS'

'in' acts as a link.