First person engaged by tense part of play left unspoken (5)
I believe the answer is:
'left unspoken' is the definition.
(something tacit is left unsaid)
'first person engaged by tense part of play' is the wordplay.
'first person' becomes 'I' (first-person pronoun).
'engaged by' indicates putting letters inside (I've seen this in other clues).
'tense' becomes 't' (abbreviation used in many dictionaries).
'part of play' becomes 'act' (a play is made up of acts).
'i' put into 'tact' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Implied -- inferred" , "Implied but not directly expressed" , "Understood, implicit" , "Unspoken but understood" , "Not said though understood" .)