First to activate the pump (5)
I believe the answer is:
prime
'first' is the definition.
(similar in meaning)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for prime that I've seen before include "put in the picture" , "Best - get ready" , "Maybe eleven" , "Best quality, perhaps of beef" , "Chief or most important, like a minister" .)