They swim in waters around the Docklands centre (5)
I believe the answer is:
seals
'they swim' is the definition.
Although both the answer and 'swim' are verbs, they are not in the same form of the verb.
'waters around the docklands centre' is the wordplay.
'waters' becomes 'seas' (sea is a kind of water).
'around' means one lot of letters goes inside another.
'centre' indicates the central letters.
The centre of 'docklands' is 'l'.
'seas' enclosing 'l' is 'SEALS'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for seals that I've seen before include "Fastens or closes securely" , "they're impressive" , "Great swimmers" , "Animals spotted on coast" , "Marine mammals" .)