Show fear when beast is taken by queen (5)
I believe the answer is:
cower
'show fear' is the definition.
(I've seen this in another clue)
'beast is taken by queen' is the wordplay.
'beast' becomes 'cow' (cow is a kind of beast).
'is taken by' says to put letters next to each other.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'cow'+'er'='COWER'
'when' is the link.
(Other definitions for cower that I've seen before include "Shrink back in fear" , "to emulate Burns's wee beastie" , "Quail" , "Show abject fear" , "Cringe, crouch" .)