Fish
answers are all types of fish:-
1, Earth, water but only one leg (9)
2,Plurally we hide around the house (7)
3,Horror, fear! It's within (4)
4,A small flat bottomed boat (4)
5,Puffin's favourite meal (4,3)
6,Eastern girls congregating (6)
7,Is it in a harp? (7)
8, .....(5secs).....Pardon?(8)
9,A con & 3.1415927 (6)
10, In the loch, a keen angler awaits (4)
11, Blair's home in switzerland(5)
12,Mother's right with nowt back (6)
13,It doesn't go round on its own (7)
14,Pay on receipt (3)
15, Earth, water & the captain (10)
16,A century without the points (4)
17, In my proposal, mondays are banned (6)
18, Hard chip with this fish (8)