Appropriate time to tuck into marine mammal? (5)
I believe the answer is:
steal
'appropriate' is the definition.
(appropriate can mean to steal)
'time to tuck into marine mammal?' is the wordplay.
'time' becomes 't' (abbreviation).
'to tuck into' means one lot of letters goes inside another.
'marine mammal?' becomes 'seal' (I've seen this before).
't' inserted inside 'seal' is 'STEAL'.
(Other definitions for steal that I've seen before include "Purloin" , "Nick" , "Creep" , "Lift - pocket" , "bargain?" .)