Never deem pet bats to be placid (4-8)
I believe the answer is:
even-tempered
'be placid' is the definition.
('even-tempered' can be a synonym of 'placid')
'never deem pet bats' is the wordplay.
'bats' is an anagram indicator (slang for mad).
'never'+'deem'+'pet'='neverdeempet'
'neverdeempet' anagrammed gives 'EVEN-TEMPERED'.
'to' acts as a link.
(Other definitions for even-tempered that I've seen before include "Placid - not easily angered" , "Calm, equable" , "Imperturbable" , "Placid, calm" , "Of calm disposition" .)