Tense husband finally wrote thank-you letter (5)
I believe the answer is:
theta
'letter' is the definition.
(theta is a kind of letter of the alphabet)
'tense husband finally wrote thank-you' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'husband' becomes 'h'.
'finally' indicates one should take the final letters.
'thank you' becomes 'ta'.
The final letter of 'wrote' is 'e'.
't'+'h'+'e'+'ta'='THETA'
(Other definitions for theta that I've seen before include "Greek letter equivalent to th" , "Greek character" , "classic character?" , "Foreign letter" , "Letter before iota" .)