wordplay - Recycling old answers

Each of the puzzles below gives a two-word answer. Solve the puzzles and arrange the resulting 16 words into four groups of four, each with a common theme. The resulting group names form a clue for the final answer.

1. Written every which way


2. A manager's dilemma


3. Keeping you safe


4. Coordinated efforts



5. Contours of a clandestine nature


6. Between two walls


7. Pulling a fast one


8. In excess I'm lethal

Answer

The four groups are:

1

London: Fire, Marathon, Olympics, Underground

2

Tower: Cell, Devils, Ivory, Water

3

Guard: Coast, National, Old, Security - thanks @arbitrahj for this one

4

Duty: Cycle, Double, Heavy, Line

The answers spell out

London Tower Guard Duty, which seems to be hinting at the Yeomen Warders, whose duty it is to guard the Tower of London.