Fixed time in animal lair (4)
I believe the answer is:
sett
'animal lair' is the definition.
'sett' can be an answer for 'lair' (I've seen this before). I'm unsure of the 'animal' bit.
'fixed time in' is the wordplay.
'fixed' becomes 'set' (both can mean decided or determined).
'time' becomes 't' (abbreviation).
'in' indicates putting letters inside.
'set' placed around 't' is 'SETT'.
Can you help me to learn more?
(Other definitions for sett that I've seen before include "paving stone" , "Retreat" , "Underground 10 [HOUSE]" , "Test for a badger's burrow" , "Paving block; badger home" .)