This is a container clue - look for indicators like 'around', 'holding', 'outside', 'embracing' where one part wraps around another
One part goes inside or around another
Answer: ETERNAL | Definition: 'Constant' | Wordplay: An envelope (‘protection … by’) of TERN (‘one aquatic bird’) in [t]EAL (‘another’ – i.e. another aquatic bird, one of various kinds of duck) minus its fist letter (‘the first flying away’).