A guide to Tom’s domination? (4,2,5)
I believe the answer is:
rule of thumb
'a guide' is the definition.
(I've seen this before)
'tom's domination?' is the wordplay.
I cannot quite understand how this works, but
'tom' could be 'thumb' (folklore character Tom Thumb) and 'thumb' is found within the answer.
This explanation may well be incorrect...
'to' acts as a link.
(Other definitions for rule of thumb that I've seen before include "Practical guidance" , "Little Tom-ocracy's" , "Ad hoc form of gauging something" , "digital governance?" , "Rough and ready practical method" .)