Specialist to strain to restrict power (6)
I believe the answer is:
expert
'specialist' is the definition.
(a specialist is an expert in a field)
'strain to restrict power' is the wordplay.
'strain' becomes 'exert' (I've seen this before).
'to restrict' indicates putting letters inside (inserted letters are restricted or trapped).
'power' becomes 'p' (symbol in physics).
'exert' going around 'p' is 'EXPERT'.
'to' is the link.
(Other definitions for expert that I've seen before include "One who knows his stuff" , "Having specialised knowledge" , "Masterly" , "Master" , "At the right moment" .)