Fortify new gents with last bit of timber, therefore (10)
I believe the answer is:
strengthen
'fortify' is the definition.
(I know that fortify can be written as strengthen)
'new gents with last bit of timber therefore' is the wordplay.
'new' indicates anagramming the letters.
'with' means one lot of letters goes inside another.
'last bit of' suggests the final letters.
'therefore' becomes 'then' (I've seen this in another clue).
The final letter of 'timber' is 'r'.
'gents' is an anagram of 'steng'.
'steng' enclosing 'r' is 'streng'.
'streng'+'then'='STRENGTHEN'
(Other definitions for strengthen that I've seen before include "Fortify" , "Reinforce, substantiate" , "rally" , "Bolster" , "Make tough" .)