Feel tense leaving specialist with niece at sea (10)
I believe the answer is:
experience
'feel' is the definition.
(synonyms)
'tense leaving specialist with niece at sea' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'leaving' indicates named letters should be taken away.
'specialist' becomes 'expert' (I've seen this before).
'with' means one lot of letters go next to another.
'at sea' is an anagram indicator.
'expert' with 't' taken away is 'exper'.
'niece' with letters rearranged gives 'ience'.
'exper'+'ience'='EXPERIENCE'
(Other definitions for experience that I've seen before include "Knowledge gained over time" , "Go through" , "Witness" , "Try" , "Feel (an emotion)" .)