An irrational terror's beginning in bed? (3)
I believe the answer is:
pit
'bed?' is the definition.
(I've seen this in other clues)
'an irrational terror's beginning' is the wordplay.
'an irrational' becomes 'pi' (an irrational number).
'beginning' suggests taking the first letters.
The initial letter of 'terrors' is 't'.
'pi'+'t'='PIT'
'in' acts as a link.
(Other definitions for pit that I've seen before include "Sizeable hole in the ground" , "Large hole; mine" , "Mine-shaft" , "Tip into miners' shaft" , "Mine; fruit-stone" .)