Volunteers on about large claw (5)
I believe the answer is:
talon
'claw' is the definition.
(I know that talon is a type of claw)
'volunteers on about large' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'on' says to put letters next to each other.
'about' becomes 'on' (as in a book 'on' a certain topic).
'large' becomes 'L' (eg in clothes sizes).
'on' put after 'l' is 'lon'.
'ta'+'lon'='TALON'
(Other definitions for talon that I've seen before include "Sharp hooked claw" , "Prey-bird's claw" , "one that grips" , "Eagle claw" , "a gripping one" .)