Getting as far as bridgehead before breaking through (9)
I believe the answer is:
breaching
'breaking through' is the definition.
'breaching' can be an answer for 'breaking' ('breach' can be a synonym of 'break'). I am unsure of the 'through' bit.
'getting as far as bridgehead before' is the wordplay.
I cannot really see how this works, but
'getting' could be 'reaching' (reaching is a kind of getting) and 'reaching' is present in the answer.
'bridgehead' could be 'b' (head letter of 'bridge') and 'b' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for breaching that I've seen before include "Breaking a rule or law" , "Transgressing, rules say" , "Breakers are" , "Breaking or failing to observe a law" .)