Fortifies at last with one additional strength (5)
I believe the answer is:
sinew
'strength' is the definition.
(I know that sinew is a type of strength)
'fortifies at last with one additional' is the wordplay.
'at last' suggests the final letters.
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'additional' becomes 'new' (I've seen this in another clue).
The final letter of 'fortifies' is 's'.
's'+'i'+'new'='SINEW'
(Other definitions for sinew that I've seen before include "Tough material" , "Muscular strength" , "source of physical strength" , "As synonym for 'strength'" , "Tendon - ligament" .)