Sailor on leave in the centre of Yarmouth is a pest (8)
I believe the answer is:
mosquito
'a pest' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'sailor on leave in the centre of yarmouth' is the wordplay.
'sailor' becomes 'os' (ordinary seaman).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'leave' becomes 'quit' (quitting is a kind of leaving).
'in the' indicates putting letters inside.
'centre of' says to take the centre.
The middle letters of 'yarmouth' are 'mo'.
'os'+'quit'='osquit'
'osquit' going into 'mo' is 'MOSQUITO'.
'is' is the link.
Can you help me to learn more?
(Other definitions for mosquito that I've seen before include "British WWII multi-role combat aircraft" , "Insect - WWII fighter-bomber" , "Malaria carrier" , "Biting bug" , "Blood-sucker" .)