Toddler's first language is brief (5)
I believe the answer is:
terse
'brief' is the definition.
(synonyms)
'toddler's first language' is the wordplay.
'toddler's first' becomes 't' (1st letter of 'toddler').
'language' becomes 'erse' (historical term for Scottish Gaelic).
't'+'erse'='TERSE'
'is' is the link.
(Other definitions for terse that I've seen before include "Sententious" , "Reset to be pithy" , "Short and to the point" , "Briefly expressed" , "Compact" .)