Rodent caught by the unusual mechanical device (7)
I believe the answer is:
ratchet
'mechanical device' is the definition.
(ratchet is a kind of mechanical device)
'rodent caught by the unusual' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'caught' becomes 'c' (cricket abbreviation).
'by' means one lot of letters go next to another.
'unusual' indicates an anagram.
'the' anagrammed gives 'het'.
'rat'+'c'+'het'='RATCHET'
(Other definitions for ratchet that I've seen before include "Set of teeth" , "Device promoting One Direction" , "One-way toothed wheel" , "Device consisting a toothed wheel and pawl that allows movement in one direction only" , "Chatter (anag)" .)