Mantra-Drsah one who knows the science of mantra

Srimad Bhagavatam 9.4.10

yat te pitāvadad dharmaḿ
tvaḿ ca satyaḿ prabhāsase
dadāmi te mantra-drśo
jñānaḿ brahma sanā

SYNONYMS
yat — whatever; te — your; pitā — father; avadat — explained; dharmam— truth; tvam ca — you also; satyam — truth; prabhāsase — are speaking; dadāmi — I shall give; te — unto you; mantra-drśah — who know the science of mantra; jñānam — knowledge; brahma — transcendental; sanātanam — eternal.

TRANSLATION
Lord Śiva said: Whatever your father has said is the truth, and you also are speaking the same truth. Therefore, I, who know the Vedic mantras, shall explain transcendental knowledge to you.