Is change in enthalpy defined only at constant pressure?
No. The definition of $\Delta H$ is $\Delta H = \Delta U + \Delta (PV)$ Simple as that.
I know Q-W=del(U) And Q at constant pressure equals del(H) (enthalpy
Can Q (heat given to the system) be used interchangeably with del(H) in the first equation?
No. Only if the applied pressure is held constant during the change.
Is del(H) =nCp del(T)? How?
No. Only for an ideal gas. For real gases, this equation is not correct outside the limit of ideal gas behavior. For an ideal gas, $\Delta U=nCv\Delta T$ and $\Delta (PV)=nR\Delta T$, so $\Delta H=n(C_V+R)\Delta T$. And then $C_p=(\partial H/\partial T)_P=C_v+R$, so $\Delta H=nC_P\Delta T$.
For a real gas, liquid, or solid,
Note that the term in brackets is zero for an ideal gas.