Say, measure bird’s speed (7)
I believe the answer is:
titrate
'say measure' is the definition.
The definition and answer can be both related to change as well as being verbs in their base form.
Perhaps there's a link between them I don't understand?
'bird's speed' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'speed' becomes 'rate' (rate is a kind of speed).
'tit'+'rate'='TITRATE'
Can you help me to learn more?
(Other definitions for titrate that I've seen before include "Carry out scientific analysis" , "Measure the strength of (a solution) using a reagent" , "measure volume" , "Use indicator maybe" , "Determine strength of solution (chem.)" .)