Was of service, but showed no haste (6)
I believe the answer is:
waited
'showed no haste' is the definition.
The definition and answer can be both related to thinking as well as being past participle verbs.
Maybe there's an association between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for waited that I've seen before include "Remained in readiness" , "Lingered; attended" , "Held back" , "Served at table or postponed some action" , "Stayed in one place, anticipating something" .)