Swimmer thanks head of diplomatic staff (7)
I believe the answer is:
tadpole
'swimmer' is the definition.
(I've seen this before)
'thanks head of diplomatic staff' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'head of' suggests taking the first letters.
'staff' becomes 'pole' (I've seen this before).
The first letter of 'diplomatic' is 'd'.
'ta'+'d'+'pole'='TADPOLE'
(Other definitions for tadpole that I've seen before include "Old tape (anag.)" , "To plead for tiny frog" , "Legless creature" , "Young Kermit" , "Young toad" .)