Truss extremely upset - to begin with only one conservative resigned (5)
I believe the answer is:
stoic
'resigned' is the definition.
(I've seen this before)
'truss extremely upset to begin with only one conservative' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'upset' shows that the letters should be reversed in order.
'to begin with' indicates taking the first letters.
'one' becomes 'i' (Roman numeral).
'conservative' becomes 'C' (UK political abbreviation).
'truss' with its middle taken out is 'ts'.
The first letter of 'only' is 'o'.
'ts' backwards is 'st'.
'st'+'o'+'i'+'c'='STOIC'
(Other definitions for stoic that I've seen before include "One showing endurance during adversity" , "Someone seemingly indifferent to pleasure or pain" , "Ancient philosopher" , "not given to complaining" , "the original Zeno?" .)