Clue: River in 15 location, centre of Luton to the west — 15 or Beds, for example? (9)
Answer: TETRAGRAM
Explanation: Definition: 15 or Beds, for example
A reversal ('to the west') of an envelope ('in') of R (the first one, 'river') in MARGATE ('somewhere in 15') plus T ('centre of Lu**T**on'). KENT ('15') AND 'Beds' both have 4 letters.