Emergency care regularly needed on part of the Thames (6)
I believe the answer is:
crisis
'emergency' is the definition.
(I've seen this before)
'care regularly needed on part of the thames' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'needed on' says to put letters next to each other (in a down clue, some letters go on top of others).
'part of the thames' becomes 'isis' (I can't explain this - if you can you should give a lot more credence to this answer).
The alternating letters of 'care' are 'cr'.
'cr'+'isis'='CRISIS'
Can you help me to learn more?
(Other definitions for crisis that I've seen before include "Time of sever difficulty" , "Desperate situation" , "Time of intense difficulty or danger" , "Climax point in, say, a fever" , "Decisive or critical moment" .)