Start with boy admitting bad speller? (6,2)
I believe the answer is:
switch on
'start' is the definition.
(both can mean to activate or initiate a device)
'boy admitting bad speller?' is the wordplay.
'boy' becomes 'son' (synonyms).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'bad speller?' becomes 'witch' (wicked woman who casts spells).
'son' enclosing 'witch' is 'SWITCH ON'.
'with' is the link.
(Other definitions for switch on that I've seen before include "fire" , "start" , "Engage an electrical device" , "Give power" , "Make contact with" .)