Introduce Polish grammatical feature (7,7)
I believe the answer is:
present perfect
'grammatical feature' is the definition.
(present perfect is a grammatical tense)
'introduce polish' is the wordplay.
'introduce' becomes 'present' (synonyms).
'polish' becomes 'perfect' (I've seen this before).
'present'+'perfect'='PRESENT PERFECT'
(Other definitions for present perfect that I've seen before include "giver is happy to hear this?" , "I have left in this?" , "writing the briefest of thank you letters?" , "Tense" .)