I am to master Gaelic in plunge (7)
I believe the answer is:
immerse
'plunge' is the definition.
(synonyms)
'i am to master gaelic' is the wordplay.
'i am' becomes 'i'm'.
'to master' becomes 'm'.
'gaelic' becomes 'Erse' (synonyms).
'im'+'m'+'erse'='IMMERSE'
'in' acts as a link.
(Other definitions for immerse that I've seen before include "Drown" , "Baptise" , "Submerge in water or liquid" , "Submerge in water as once for baptism" , "Dip totally into liquid" .)