The phrase "to instruct a solicitor" means to hire and give directions or guidance to a lawyer or legal professional for legal advice or representation in a specific matter or case. Full definition
They have already instructed a solicitor who has spent quite a bit of time with them and who has their trust so they can not see the point of mediation. (compass-resolution.com)
You will normally instruct a solicitor to act on your behalf when you're buying a property. (sainsburysbank.co.uk)
If you already have legal expenses insurance cover you can still instruct the solicitor of your choice. (sds-solicitors.com)