With regard to tackling United, become hardened! (5)
I believe the answer is:
inure
'become hardened' is the definition.
(I've seen this before)
'with regard to tackling united' is the wordplay.
'with regard to' becomes 'in re' (I've seen this before).
'tackling' is an insertion indicator (to tackle is to take possession of).
'united' becomes 'U' (abbreviation).
'inre' going around 'u' is 'INURE'.
(Other definitions for inure that I've seen before include "Harden or season" , "Cause to become hardened to" , "Temper, toughen" , "Steel" , "To give experience" .)